There are over 900 MLS organizations in the US. Their purpose is aggregating the RE listings in a region. This was super important 100 years ago, still pretty important 25 years ago, and now has little utility for consumers now that listings are aggregated online and searchable without an agent across regions with better-than-mls-portal consumer-grade shopping tools.

Today they essentially function as data brokers and unofficial labor organizers, collecting membership dues from agents in exchange for ensuring that the agent's name is on their listing wherever it's shown online and (along with NAR) pushing agents to keep "standard" commissions on their listings (5-6%).