We warlocks have a litany of curses at our disposal, with wildly varying effects and durations. Fortunately for our targets, we can only land one curse per target, whether it's a straight-up DoT like Curse of Agony, or a debuff effect like Curse of Tongues.
Much of the Warlock's group utility and versatility are defined by our access to these various effects. While some curses have wider applications than others, it is hardly a one-curse-fits-all situation. Knowing which curse to use in the right situation is key to maximizing your Warlock's potential.