The question should be: "why WOULDN'T you hack". Pokemon is incredibly boring after the main story if you want to play competitively (aka serious play). You have to breed, EV train, check nature, etc...just to get a perfect Pokemon. And after all that training, that team might not even WORK because after all the time you took, there are new trends in OU or the tier you play in. Or you might just straight up decide you don't like the team after testing it. That's why pokemon simulators are so popular, and tools like PkHex are a godsend.
This is for speeding up the process, not cheating. Hacking is not always cheating.