First it was a matter of fun vs price. that or if I could trade it/sell it to fund a purchase to build my collection.
Then it became all about the rarity. Was fun to collect what others don't have or what you never get to see. You learn a lot by working on games that not many know about. Learn to make your own parts too or track down things that are considered unobtanium.
Now I still go after rare stuff, but the list is narrow since I have most of what I want. Eatpm is the only thing on the docket to let go of, but its been fun building up through trades. Started with a fire damaged and gutted LAH which I sold to buy a rbion with issues which I traded for eatpm.