There are many reasons why people collect, the same way there are many reasons why people have the personalities that they have.
My wife and I enjoy watching Antiques Roadshow and a few other TV shows about collecting.
What you hear time and time again from the "experts" is that you shouldn't collect for the future value but to collect "What you like"... what brings you joy when you see it or play with it.
I'm no psychologist so I can't give you the reasons, but I do have a bunch of little collections of stuff that really brings a smile to my face and makes my heart light. They are really only worth something to me. (Like your couple of State cars Pete.) Maybe they might be worth more in the future but that's just the ol' icing on the cake. A different motivation for every one.
Fred
PS As I've stated before I'm a little, shall we say, frugal with my funds, but I have almost exclusively Micro-trains cars. I collect anything CN, CP, ALGOMA, TH&B transition era but also pick up any cars that interest me aestheticaly or has some meaning: Like Susy Q, or the St. Louis "Blue Flash" (reminds me of Elvis and his "in-a-flash/TCB"), or a Maine boxcar 'cause that's where my sister lives... even a car from the Rock Island Line 'cause I really like Johnny Cash!