1)to provide the party with a lot of SP. I had fun with 4D killing giants with fireball in LoG.

2) to heal paralysed and stoned friends. It's really nice to provide one character with protection aganist stone and paralyze (In MM7 the cloak of the sheep is really nice to have.) and then cure victims of those spells instead of finding items of protection for both the cleric and the sorcerer. It's also nice that one "person" can concentrate on mind and earth magic early in the game and leave the sorcerer free to learn water.

3) To mix white potions for challenges. Druids can put a lot of skillpoints into alchemy and meditation. Sorcerers and clerics needs those skillpoints for other things. Being able to mix black potions is a nice bonus.