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...
Not exactly stellar examples here. A paladin multiclass can smite a little bit more, but hardly "all day long" and gives up some really nice paladin class features just to squeeze out a few more smites per day. A spore druid multiclass for constitution saving throw proficiency is ridiculous - you might as well use the feat you would have lost from the multiclass on Resilient: CON.
Most multiclasses I've seen people rave about suffer from a lack of consideration to the class features lost by multiclassing. Multiclassing for most people is a method to either knowingly or unknowingly de-optimize their characters, which is fine since BG3 really doesn't require min-maxing. There's an extremely limited number of beneficial multiclasses; the rest are surpassed by the standard classes. Obviously, you should multiclass if you feel like it's fun, but don't be surprised when your multiclass starts to feel like a glass cannon or one-trick pony compared to any of the regular "boring" single class characters.
