When I was a kid I was fascinated by the idea of dragons and dinosaurs. I think most kids are, but these ideas and images retain their fascination for some of us more than others. As for the why of it...well these images fire the imagination when you're a kid, but there are deeper ideas buried in the fossil record, regarding the nature of life and death on this planet.
The reptiles and amphibians I keep remind me of a much larger, more complex natural world. They're also aesthetically pleasing - living studies in line and colour, and they're profoundly different from me.
Personally, I don't keep them to educate or enlighten others. I enjoy working with my dart frogs or phantasticus the way someone would enjoy working in their garden. It's simple enough, I think.