I think the most obvious reason to believe things are conscious (and indeed for consciousness to have evolved at all) is it’s a very good way to predict the behaviour of others. If I want to know how predator, prey, pack member or even my own child will react to a situation I can use my own conscious experience to guess their next moves. For social mammals this would be highly advantageous. Of course this could just be me inventing a just so story. I’d love to see if it’s more than that.
Within the goal of maximizing one's own life quality, it is sometimes useful to believe that some entities are conscious. The property of phenomenal consciousness in others is unfalsifiable; therefore, whatever belief one has about the phenomenal consciousness of some other entity can't be false. This opens up the opportunity to strategically optimize this belief. There are pros and cons to believing that something has qualia. The belief that X has qualia allows one to associate with X, feel love, gratitude, and other positive feelings and emotions. Simultaneously, it also makes one suffer from observing X in pain and gives X the power to emotionally manipulate you. Other pros and cons can be discovered through experiencing life, but these are enough to outline a sketch of a strategy: one should believe that X is conscious if it gives more positive emotions than negative ones. I believe that my employer is conscious because it gives me more energy to work, leveraging my feeling of gratitude toward my employer. I believe that those who are trying to emotionally manipulate me are not conscious, because this gives me immunity to the manipulations. I can strategically change this belief from time to time. I can temporarily believe that a lamppost on a street is conscious if I want to express care for that lamppost.