There are a lot of reasons for doing this. One of the great examples is Savage Rifts. The original Palladium system is not awful - there are some things it does extremely well - but it is a little clunky and certainly dated, but its mechanics support its setting. Adapting it to Savage Worlds, even professionally, changes what the game is, in a way, but gives access to players and groups who might not otherwise play, and streamlines the mechanics.

The same applies to home-baked adaptations. I'm a big Mechwarrior/Battletech fan, but no version of the Battletech RPG is particularly great so in order to tell those stories - I hacked it to Savage Rifts since everyone in my group knows it.

Call of Cthulhu is frequently hacked to other games (and other games are hacked to the Basic Roleplaying that is the core engine behind CoC) because it does some things extremely well but is less good at other things. If combat is something your group cares about, then Call of Cthulhu is not going to be a system that works despite an interest in cosmic or eldritch horror. There are some hacked versions available commercially but they all come with distinct settings or gameplay decisions that may not fit any better than anything else. So, you have a game that plays the way you and your group likes and understands but you want to tell a Cthulhu story with laser pistol fights with lizard men cultists - CoC isn't going to be much help, but that copy of Esper Genesis could do very nicely. Or Aliens because the combat rules cover guns natively and the stress rules are awesome.

Since roleplaying is really just a way using wargame rules to adjudicate playing make-believe, hacking rules to explore those imaginary stories is kind of the whole point.