Not sure why you were downvoted for this. I mainly use it out of curiosity as well. It's very occasionally useful as a sales tool ("Hey, look, your competitors are all using Magento so you should too!") but I'd be lying if I said I used it that way on anything like a regular basis.
HN has become a tough crowd to please these days...
If folks demand a complete answer from me, then here is the super dull list of reasons I use BuiltWith:
1. I have clients that ask me to check out XY thing on a competitor site. A lot.
2. I'm curious about tools other people use so I can learn the tech and recommend for clients
3. When bidding on a project I need to check what platform it's on
4. Doing market research for a side project (assessing e-commerce sites in a particular niche)
