A protectorate bundles an alliance with military access between both parties. If they pay any tribute to you automatically, I don't see it. Most important, however, is that the territories you act as patron to their client count as yours for victory conditions. So, when you get to the far side of the world, where you can't maintain order without massive garrisons and huge expense, make them a protectorate instead. You don't get the income or production from those cities, but you don't have to spend money on upgrading the cities, worry about them revolting, or maintain garrisons either.