I have demons/devils who can grant spells, and I still need evil gods.
The reason is for 'popular evil.' It's kinda like Loki vs. Demogorgon....Demogorgon is an exception, an abberation, a being once mortal now semi-divine, whose agenda and goals are thuroughly selfish and desctructive, with no care for any good save his own.
Loki, however, is an integral part of the pantheon. He's related to half of 'em, he's helped a few out, and, despite his wickedness, he's still relatable. He keeps the world around, and he keeps the wilds dangerous. He works within the 'deity system' despite being one of the key destroyers of that system.
Demogorgon, or a planar lord would never ever be useful in any way beyond their own desires. Loki, on the other hand, is beholden to the nature and politics of being a deity.
Maybe think of it as the Madman and the Demonic Emperor. The Madman obliterates all that he sees, for he feels no mortal pain, and fears no mortal foe. The Demonic Emperor is relatable, he's got a logic (albeit an evil one), and a corrupted form. If he destroys the world, there is none to rule over. They're both Chaotic Evil, but it's a matter of 'pure'ness. The Emperor is less pure, the Madman is less relatable.
Basically, I need evil gods for the same reason I need evil humans, evil elves, evil gnomes, evil dwarves...because it's somehow slightly more disturbing when the wicked deities are related and recognizable, as something anyone could become, rather than some alien corruption from beyond. Not all flavors of evil are the same extreme. Some are much, much more close to home.