It stops the compiler from complaining about unused variables:

http://stackoverflow.com/questions/8052091/void-cast-of-argc...

Why would you do this instead of declaring it as `int main(void)`?

I use clang with a -Wall and -Wextra, which will complain about all sorts of stuff I generally want to know about, including unused variables (I think gcc does the same?). For the rare cases where I actually want to leave a variable unused I do the above to make the warning go away.