While combat sport isn’t real combat, it can’t be brushed out as completely irrelevant (as it done at times). Fighting sport training provide practitioner with good physical conditioning and base physical (such coordination, balance, flexibility, etc) and mental (sense of opponent, ability to withstand pressure) skills that crucial in combat and self-defense training.
Some of those skills can be developed by practicing any (not necessary combat) sport, but some are specific to selected combat disciplines such Judo and wrestling or boxing, Muay-Thai, karate, etc. From self-defense perspective training good striking skills isn’t as complex task as developing grappling skills because self-defense striking can be trained individually, but grappling requires matching and diverse training partners.
Not all types of combat sport are equally good as a base for self-defense training, but all will give you basic skills.
Sport, combat, and self-defense