I feel like the bigger reason to have evil races is to have a more or less ever present challenge and point of conflict. For instance, the underdark is horrible place to be, in large part due to the drow. Their presence and general alignment of evil makes the setting dangerous and interesting. Is this town safe? Have the drow been messing about assassinating local leaders? Should we help this group by liberating them from slavery from the drow?
It's almost like their species is in of itself a character, with this species sized character being evil. Having an entire species be generally evil gives the world more scale than a single evil character would. But yes, an individual villain needs more than just their evil race to be interesting.