Why do we have antitrust laws?

The Supreme Court has called them "the Magna Carta of free enterprise", United States v. Topco Assocs., Inc., 405 U.S. 596, 610 (1972), and the Sherman Act "a comprehensive charter of economic liberty", Northern Pac. R.R. Co. v. United States, 356 U.S. 1, 4 (1958).

Sounds great. But