But to be honest I started before then, on the ZX Spectrum. First of all it was patching games to get infinite lives, or time. But later it became necessary to patch the loaders before you could even access the game-code - speedlock, bleeplock, etc.
Being able to pause a running game and peek/poke at the RAM would have been very useful for hacking games, though of course I'd still need to crack the loader to share the POKEs with other people.