I have often considered why people collect, and have come across very few explanations that are satisfying to me. Mostly, articles on the subject end up simply talking about THINGS one can collect, or talk about profit motives. That doesn't interest me. I am interested in the underlying reason for the gratification of collecting. Obviously there is some sort of gratification if one does it for fun.
Coins, stamps and the like can be "completed", and so I suppose the goal of completing one's collection is gratifying in a sad way. But butterflies or rocks can't be completed, so the joy there must be in accumulating the most examples of variations and varieties.
What about people like me who like BIG things? I like old steam locomotives and ocean liners. These things are hard to collect (and even harder to store, by god) especially if one is a small dog on a limited writer's income. This obstacle is easy enough to overcome by just collecting images of steam locomotives and ocean liners. Photographs, postcards, old advertisements, company propaganda. When I visit museums which contain steam locomotives (Henry Ford was an obsessive collector of things, and his collection ended up in the Henry Ford Museum, even huge locomotives and other mighty things with huge flywheels, like steam dynamos) I use the opportunity to take pictures of them. Mostly, though, I collect picture books of these things.
The final resting place of the chair Lincoln was sitting in at the theater when he was offed, and (coincidentally?) the limousine Kennedy was riding in when HE was offed are also in Henry's magnificent collection, the latter collected on his behalf, apparently, long after his death. Henry Ford also collected houses and buildings and had them transported to and reconstructed at his Greenfield Village, out behind his museum in Dearborn. Churches. Edison's laboratory. Ford's childhood home. An ancient carousel in a building. A glockenspiel Henry apparently got a kick out of watching. The bicycle workshop of the Wright Brothers of airplane fame, brung on up from Dayton. Henry liked a lot of things. Most collectors don't have as much money as Henry Ford had, though.
Psychologists say part of the urge to collect is the desire to stimulate pleasant memories. I don't know. That doesn't explain why some people collect war memorabilia. If you collect souvenirs and brochures from past vacations, I suppose the reason might be to stimulate past pleasant memories. If the trip were pleasant.
Some people collect ticket stubs, autographs, old musical instruments, postcards, vials of dirt. I once read about some guy who claimed to have a vial of dirt from every state. Well, jesus god, you know. How does one prove that, I'm thinking.
Some people collect sea shells. My mother used to collect stones. She didn't collect them because they were an example of this or that geological variety, or because she was completing a set. She collected them because they were pretty to her in some way. That is as good an explanation as I've found: collect because the object brings you joy.