It seems common that people build chars that compliment themselves. Like air/water and fire/earth wizards. But when they do so it takes 2 turns to get the effect you want, and the enemy moves around. So better to have one char be able to setup, and another to unleash. Like this you can do it in one turn, a wizard chasts oil, and a ranger uses a fire arrow for example.
Its the same thing with a buff like Oath of desecration, if your warrior or rogue is casting it on himself, that is a whole turn out of 3 or so that is spend. If another char casts the buff the damage dealer can get full use of it. Same with haste, you will often want someone else to cast it to be able to use it in the same turn.
