i'm truly sorry to bring this up but since you mentioned it in your description, i did always wonder why people would buy drawings. I'm not trying to ask this to 'troll' or provoke you or anything, I really just asking to try to understand why, I've tried to think about it but I couldn't find a logical reason.
I believe it's about character design. What you're buying is concept art from a concept artist for you to use in your final product, which is often artwork or stories. Makes sense. Though I agree, I wouldn't buy concept art because I tend to make my own.
Because the character shown is what you get. You get to use her in whatever way you please. People that bid on adoptable auctions generally do so so they can get characters for a manga they're working on, a video game, etc.