The GNU manpages are useful for programmers; first of all, you can lookup commands that you might use while programming, in case you have forgotten an option or something like that, but additionally, they contain documentation for many C functions, so you can for instance run man printf to get information on C’s formatted output function. Also, GNU/Linux kernel calls are documented.
You can also run man ascii to get a manpage containing every ASCII character in the set:
There is no need to open up Firefox or Chrome/Chromium (but dare not use Internet Explorer) each time you need some of that information, just use the manpages (especially useful if you happen to be without an Internet connection).