So that they understand how a network mask works? I'm assuming if you're working on the web you ought to know something about networks, right?

Why would a regular web developer need to know anything about network masks? Those come up in administration, but generally not in development.

I might be asking a bit much here, but I generally expect someone who's developing something that's going to run on the Internet to have some understanding of how the Internet works. Like... you don't need to understand the intricacies of BGP, but some idea of how a request from your web browser leaves your home network and comes back is strongly appreciated.

My biggest reasoning for desiring that skill is debugging. If you build a system and it doesn't work right for whatever reason, it's good to understand why that might be. Also, having some appreciation for that stuff really helps you make good decisions (but why can't I make 200 HTTP requests while loading my page?!)