Frame pointer omission does make debugging significantly harder. Local variables are harder to locate and stack traces are much harder to reconstruct without a frame pointer to help out. Also, accessing parameters can get more expensive since they are far away from the top of the stack and may require more expensive addressing modes.
The -fno-omit-frame-pointer option direct the compiler to generate code that maintains and uses stack frame pointer for all functions so that a debugger can still produce a stack backtrace even with optimizations flags.
Irrespective if you use the flag, not every function needs a frame pointer in the first place, so you can't always expect a difference with the flag.
Also, whether the function has a frame pointer is an implementation detail. Compilers can differ in implementation details (and usually they do).