one player quests have some stuff you unlock like turning enemy parts into weapons or getting soul eater and a fry pan but no nothing in the game really needs to be done alone.
Pretty sure the tower quests and ep4 side projects can be done in multiplayer mode as well. So you can have people help you unlock s-rank weapons, %s added to weapons, and hit % added to enemy weapons. And I know Claire's Deal is a multimode quest as well.