I think it's partly to control difficulty better. In BG if you build your party neglecting some key concepts (a dps, a tank, crowd control spells etc) you can v

I think it's partly to control difficulty better. In BG if you build your party neglecting some key concepts (a dps, a tank, crowd control spells etc) you can very well find yourself unable to continue at certain challenge (i.e. the for golem room if you focus too much on hth, out the first dragon in icewind.. memory about the specifics faded now pardon any confusion) and basically you do well up to the challenge then you can't progress anymore. Also a smaller party with less variations in roles is much easier to do storytelling with.