Or if you think [this game] is TOO easy, than wtf do we need a 5th or 6th party member for?
For fun? So we can more easily interact with multiple companions, especially if Larian does a DOS2 and kills off all the ones we don't choose? To have more flexibility in party compositions? So that more time in combat is spent playing the game instead of waiting for enemies to take turns? Any of the above reasons alone is enough to want a 5- or 6-person party.