Well if you look at the pantheons as part of the D&D toolkit to build your world, it gets easier to narrate. Leave out the ones you dont want in any given campaign.So D&D has had sort of two traditions about fiend lord type beings.
1 They are flat out deities, they have clerics.
2 They are explicitly not deities, they do not grant spells to clerics.
With a bit of overlap where only some are deities and some are not.
If you go with them as deities then the original question of why have Erythnul the god of Strife in Greyhawk when you have demon lords becomes why have deities of the demon group alongside deities of the devil group alongside evil gods of the Oeridian pantheon in Greyhawk alongside evil gods of the Suel pantheon in Greyhawk, alongside the Orc pantheon, etc. all in the Greyhawk cosmology.
The desire seems to be to have a world cosmology with multiple different possibly overlapping groupings of evil deities. This is a bit tautological but just wanting a multiple pantheon world similar to how for Conan's Hyperborean world the evil Stygian god Set is not a part of the pantheons of the northern Cimmerians or Picts who have different evil gods and there might be demons and Cthlhulhu mythos beings in Hyperborea as well separate or connected to the gods.