Why is "dev-mode?" a function?

[chmllr]

After reading the API documentation I didn't realize, that dev-mode? is actually a function, so I spent a lot of time, trying to understand what's wrong with my code, until I looked at the implementation. Apparently, there is a specific reason for dev-mode? being an argument-less function, but what is it? The mode seems to be set once at the server start, so why to re-evaluate the expression again and again?

Thanks!