Ah, the old palette swap huh? It was born out of having limited budgets and resources to make use of in game development companies. Outside of those reasons I think it can serve a purpose in a game to denote monsters of a similar family relation. It can give the player some ideas of tactics they need to use to overcome the monster if they have fought a monster in the same family before. For example a "slime" monster being resistant to physical attacks or being slow to act. Typically with palette swaps you also change the elemental alignment of the monster to.