As mentioned in the other thread, I currently only have one small glass cabinet's worth of display while I'm in between homes (1.5 years later ). Many of my reasons have already been said:

1. They're beautiful in form and function.
2. They remind me of a fond time in my childhood.
3. Some are things that I always wanted as a child, but couldn't have, so those are the completion of a goal.
4. I really enjoy setting up and changing the displays. It's like having my own art gallery where I curate different shows periodically.

I love showing others too, although that's just a bonus side effect, not a motivator. It's been a shock for a few ("OMG, you collect Transfomers!"), but I'm not shy, or shamed, so that doesn't bother me, and even the shocked ones come around to enjoying it after having a gander. Many of my peers have similar memories and love having a chat about it and some others even enjoy a play.

Lastly, my kids love them as well. Granted, they have lots of domestic and more age appropriate releases to play with (they're not allowed in the cabinet ), but they still love pointing through the glass trying to name as many as they can, although for my daughter, they're all "Wobot!"