Because it is fun and adds flavor and depths to characters.
I have no idea how it is in tabletop, but in BG3 you use multiclassing to mitigate weaknesses of other classes. Paladin for example is an incredibly frontloaded class with divine smite, lacking spell slots. Add a full caster class and you can smite all day long. Spore druid has lots of amazing concentration spells, but no constution saving throw proficiency, add a bit of fighter or sorcerer and you get a better druid...
