I've found that it gives me something to fall back on when I can't get parts of my Lisp code to run fast enough. :-)

More generally, though, it forces you to deal with things on a more elemental level, closer to what the machine is actually doing (at least for machines such as they are nowdays). C does very little for you compared to other languages (i.e. higher level languages). You get to have all the "fun" of dealing with memory management, pointers, etc. Learning all this has great pedagogical value. :-) It also will give you great appreciation for the people who have written nice things like garbage collectors for you.

Of course, it's useful to know. Many OSes are written in C; many available libraries are C libraries. Even if you're not using C directly, there's a decent chance that you might want to interface with such, and so knowing about C can be useful for that.

That all said, I can't think of too many times when I'd want to write anything in C. IMHO it's better off to leave it for when you actually (demonstrably) need it.

As others have said, Kernighan and Ritchie is the canonical source for learning C