Like a lot of DMs, I find that evil cultists are a pretty good intelligent villain to use in RPGs: "This guy is trying to summon Orcus to destroy your hometown! You are completely justified in stabbing him!"
For most cultists, it's a pretty miserable life, wearing hooded robes and hanging around in sewers and caves, neither of which are great robe-wearing places. Sure, you get access to those neat wavy-bladed daggers, but what's the appeal otherwise?
Without this getting too real-world (and therefore very depressing), what's in it for the cultists?
So, first of all, it's important to understand that a fantasy setting all celebrants of any divinity are "cultists". Each gathering of worshippers is a cult, whether of an approved religion or an unapproved one.
So you generally need to answer this question for any deity of your campaign world. Why bother? Why the commitment?
In the case of the evil deities, the answer in my campaign world is basically, "They are much easier to persuade to help you." Where the good deities demand you meet a rigorous standard of conduct for a lengthy period, the evil deities are much more straight forward. They are just asking, "What are you willing to sell me?" This is very appealing to the desperate, the despairing, the people who feel they've been wronged, or who feel they aren't being heard. Patience is a virtue. It's part of that whole rigorous demand on your conduct. What if you want something now? Well, you can sell something, your blood, your identity, your soul. And the payoff is immediate and visceral.
It's important to keep in mind that evil is often superficially attractive. The evil deities of my world tend not to look like those of say "The Forgotten Realms" or the Chaos Gods of Warhammer which are designed to be just obvious bad guys. They are obviously bad, but not offering portfolios of just "Sign here for death and suffering." They offer knowledge, wealth, power to overcome foes, pleasure and self-affirmation. Of course, in the fine print things might be just as bad as if you were signing up with Nurgle or something, but the point is that they are making sales of short term versus long term. You get what you want in the short term, and they get something in the long term. It's generally a bad deal, but people are bad long term planners.
One of the cults that showed up in my last campaign were Nuati the Storm Lord, who is worshipped in a propitiatory manner to avert his wrath, and who is also a fertility god, promising the sort of things you'd expect of a fertility deity - offspring, pleasure, and profit. But he's basically Cthulhu with better PR, or perhaps Poseidon Earthshaker with a bit worse PR.
