The point of antitrust legislation is to prevent monopolies from forming.
The late 1800s and early 1900s were a famous period for the use of antitrust in the United States — railroad, telecommunications (AT&T), oil companies (Standard Oil) were all broken up and forced to divest in chunks of their business. This lead to broad benefits for consumers because they stopped getting fucked over by massive companies that had no reason to get this big.
Since then antitrust has floundered. There are broad calls to enforce market competition in the tech industry.