It doesn't help as much as you'd think. With a buffer overflow on the stack, you can overwrite the stack's return address to point wherever you want, and overwrite a bit further to set the arguments to whatever you want, as well as any local variables on the stack. That depends, obviously, on what code is already there, but there's a lot you can do. For instance, if the system() libc function is linked, you can overwrite the return address and arguments to call system("some arbitrary shell code").

Check out https://en.wikipedia.org/wiki/Return-oriented_programming for more information.