The importance of Demons and Devils depends on the focus of the D&D game. I have run campaigns which did not use either, so they are definitely not strictly necessary.
However, in most of my campaigns, I have had demons as servants of powerful evil enemies and as shock troops, and I have used devils as tempters and high end evil leaders/lieutenants. I think that they are an iconic monster type, and sometimes it is nice to have unambiguously evil creatures for the party to fight.
However, in most of my campaigns, I have had demons as servants of powerful evil enemies and as shock troops, and I have used devils as tempters and high end evil leaders/lieutenants. I think that they are an iconic monster type, and sometimes it is nice to have unambiguously evil creatures for the party to fight.
