Suppose you'd written the same post about whether buying popcorn creates popcorn:

When I buy popcorn, the trade doesn't actually create any popcorn, and the conclusion seems pretty weird if you think about it. Namely, you are saying that just by making some trades that at the time don't create any popcorn, you end up creating some popcorn.

The popcorn argument is mistaken for exactly the same reason. In fact people buying popcorn causes people to make popcorn, by driving up demand. The mechanism is not speculative or mysterious. And people buying investments causes people to make investments by the same mechanism.

Another way to think about it is to think about loans rather than equity, you might find these less confusing. If I loan someone $10, and get paid back $11 tomorrow, it's not crazy to assume that they now have more stuff than they had before, otherwise why did they pay me the $1? (I might have instead duped someone into taking out a loan that they'll regret, in which case I've transferred money from them to me, but if you are saying "most of the time when people buy something, it makes them worst off" then I think you are the one with the weird claim.)

Now what if you loan someone money, and then I merely buy the loan from you, so that I give you money today and then I'll get paid back instead of you? Now am I doing anything?

Yes, because you were going to sell the loan to someone else. I've increased the number of people "buying loans" by 1, which means that I also need to increase the number of people "selling loans" by 1, i.e. I need to increase the number of loans by 1. (This isn't quite right---I can also raise the price of loans and thereby discourage another buyer, but the basic analysis ends up nearly the same.) It's just like buying popcorn from someone who doesn't make it themself, in a world where popcorn is non-perishable. It's all the same.

Loans and equity are very close substitutes, the main difference is the risk profile of repayment (loans pay a fixed amount, equity pays an amount that depends on how well the company does).

If you're willing to buy or own equity in some particular company, your demand is driving up the price for that company's equity. That company may own some of its own equity, or can issue new equity, and can then sell it at a higher price, or use it to pay for things (e.g. employee compensation, deals with third parties) directly. As a result, they can acquire capital to do whatever kind of presumably-profitable endeavor they can think of, presumably creating real economic value (or else it shouldn't be making a profit.) Your trade returns and dividends are a portion of that value. At least, you know, in theory.

If I earn some money and give it to (what I take to be) an effective charity, then as far as my charitable activity is concerned it more or less doesn't matter whether my money-earning has added value to the world or merely transferred resources from someone else to me -- because I am giving the money to an effective charity and they probably wouldn't have.

Put differently: If you believe that donating to effective charities does much, much more good to the world than almost anything else one could do with the money, then most likely the dominant effect of any transaction that yields more money for donating to such charities is the charitable effect, and other things like gains from trade are negligible.

So, long-term investment. It may or may not do the world any other good, but at least it has the effect of transferring money from elsewhere to your fund, and from there to an effective charity. If your choice of effective charity is good, this should improve the world considerably via what the charity does, and in comparison it may scarcely matter what other effects there are.

(Some people think the EA idea that some charities are super-effective is all wrong, and some of those people think that investing in almost any company is better because businesses improve the world. If they're right, then of course the most important effects of that long-term investment will be those of whatever it's invested in. But if they're right then you shouldn't be thinking about investing to give to effective charities in the first place.)

it's not clear making $1000 by investing in the stock market means that you have added $1000 worth of value to the world

No, investing $1000 creates a lot less than $1000 of value, but, usually, the value is more than 0 (though it can be 0, or even negative).

Are long-term investments a good way to help the future?

I'd be suspicious of this. If I spend the $1000 on popcorn, rather than investing it, the $1000 doesn't evaporate, it merely moves to someone else. And then that person can do something useful with it. I don't know why this should create less value then long term investment, but then my Economics 101 is lacking too.

Curiously, from a civilisational perspective it doesn't matter whether you invest the money or just stuff it in your mattress; either way you're creating capital relative to the alternative, which is to spend it now on some form of consumption. (Note that, in this view, charitable giving is a consumption good rather than a capital good. In practice, of course, it's more complicated, because Africans with mosquito nets are more likely to generate endogenous economic growth than Africans with malaria. But to first order, saving African lives is that-which-you-value, rather than a means of producing more of that-which-you-value; ergo, it's a consumption good.)

The key idea I think you might be missing is that capital is deferred consumption. That is, whenever we postpone our consumption from time X to time Y, we create capital that exists for the interval [X, Y). If we invest the money, this is obvious: the factory built with our investment is a capital good, and the profit we make depends on the stuff the factory makes being worth more than was invested to build it.

But suppose instead of investing the money, we just bury it for ten years. During those ten years, there is less money in circulation than otherwise, which drives up the value of money. I.e., it drives down the prices of goods. This means that all other holders of money now have access to slightly more capital, and can fund larger investments with the same number of banknotes. The capital is still created, and still produces a dividend, it's just that that dividend is now paid to people who are actively investing, instead of to us.

How can this be? Well, how did we earn the money in the first place? We produced some good or performed some service, and in return received only these green pieces of paper. If we just sit on them, they are sterile to us; but we still performed the service. Essentially, we are only paid for our work when we spend our wages on consumption goods (or services); it is in those goods that we are really paid. So burying our money for ten years is waiting ten years to present our claim upon society to be rewarded for the work we previously furnished. If that work was to build someone a factory, then for those ten years the factory is producing goods for a society that hasn't paid for the factory yet; if that work was to supply someone with consumption goods, then just follow the chain of substitutions / opportunity costs until you get to a capital good.

Of course, for this to work, someone has to be investing in actual capital goods; and the more people are doing so independently, the more loci of initiative are searching for profitable opportunities (and the fewer possibly-irrational trades you have to follow before hitting a capital good). So investing the money does slightly improve the system's efficiency at finding and making the best available investments. But it's the earning the money, not the investing it, that creates the value which, by deferral of consumption, becomes capital. (Thus, investing unearned income / ill-gotten gains does not create capital!)