When you get right down to it, how many of us actually need any airgun? Some of us undoubtedly live in rural areas and find them somewhat necessary for pest control, but aside from those individuals most of us just have them for target practice, hunting or as part of a collection. But even those individuals that regularly need to take out rats in the barn or rabbits in the garden would still do perfectly fine with only one or two airguns. No one ends up with two dozen airguns because they actually needed them.
Bottom line is that we're doing this just because it's fun. For some reason our lizard brain likes it.
Now with a powder burner I would argue that a pistol is actually the most essential gun that you can own for self defense. Unless you're a soldier in a war zone you're not going to be able to carry a rifle with you most of the time. So if you do take the possibility of having to defend yourself with a firearm seriously then being proficient with a pistol is definitely a skill you want to have and plenty of practice with air pistols is one way of developing that skill.
Myself, I just love to shoot, but what I don't love is driving two hours out into the national forest and then shooting on a clearcut with a 40 degree slope to it or driving 1/2 an hour to the local range and putting up with the gangster wannabes or the range nazi that's there to keep them from doing something stupid. With airguns I can shoot every weekend and in my basement whenever I want and I don't go broke or deaf in the process.