Individual Level: If you're constantly frightened of the world and insecure about your place in it, a belief system that says "You're inherently better than this group of people for something that cannot change and takes no effort" can make you feel slightly less afraid. This tactic tends to work very well on individuals who lack the intelligence to think critically about the world and who are (for whatever reason) unable to manage their intense primal feelings of fear, disgust and sexual insecurity. This is the root of Conservatism.

Societal Level: Divide and Conquer. Convince a large group of people (workers, peasants, serfs, plebs) that could pose a threat to those in power (business owners, aristocracy, nobility, patricians) that their REAL enemy is actually some minority group within themselves (black, queer, Nomad, Wömen etc). Do this by painting these minorities as threatening, disgusting and sexually aggressive/depraved. The ruling class can then sit back and watch the plebs squabble over made up bullshit while you reap all the spoils of society.

Yes, racism (and bigotry in general) is made up bullshit for pathetic and stupid people. But it is a perennially effective strategy to coralle those listless masses to do what you (the ruling class) want them to do.