Jam is a very natural way to preserve fruit. The fruit is ripe only a small fraction of the year, you'd like to be able to eat it all year long, so you cook it with enough sugar that it won't go bad for a long time. But now that we have freezers, jam is obsolete.
When you make jam, you change the taste and texture of the fruit in several ways:
- Squishing the fruit makes it into a more consistent texture, suitable for spreading.
- Adding sugar makes the jam sweeter, and also much less healthy.
- Heat changes the flavor of the fruit, With more volatile flavors evaporating.
- Heat evaporates some of the water, thickening it (reducing).
- Heat releases pectin, or you add additional pectin, which gels the jam.
Some people make freezer jam, which skips the heated steps, and gives you a flavor much closer to fresh fruit. Because it hasn't been cooked thoroughly, you need to store it in the freezer, but since freezers are a thing now, that's not a problem.
We can go a step further, though, and just use frozen fruit. Take frozen raspberries, defrost them, and squish them up with a fork:
on eggy waffles, with nutella
This takes about a minute for defrosting, 10s for squishing, and is far superior to jam. Not so sweet, better flavor, and it's cheaper  and healthier as well!
Being able to store traditional jam in the refrigerator once you've opened it is an advantage, but I think it's a temporary one. Over time, I expect households will shift towards more freezer space relative to refrigerator space, as the quality of frozen food continues to rise. We got a chest freezer soon after we bought our house, and I'm glad we did!
 Our grocery store has raspberry jam for $2.75/lb, frozen raspberries for $4.39/lb, and jam is ~40% fruit. Though since we have the freezer space like raspberries a lot I've been buying them online, 30lb at $3.08/lb shipped. A shipment fills 6-8 one-gallon ziplocs depending on whether it ends up mostly whole or mostly pieces.
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