The European Commission has fined Mastercard for stifling competition. This behavior of Mastercard had made consumer payments more expensive. Although antitrust law is not the same as criminal law, the reasoning behind punishment from criminal law can be informative here as well. There is more to it than just the money in the state treasury.
In criminal law there are different reasons for punishment that more or less also apply to antitrust punishments. First, society has been harmed and amends ought to be made. Second, the punishment is expected to reduce the chance of repetition by the offender. Third, punishment is assumed to scare off others as well. Lastly, further conditional punishments can shield society from even more repeat offenses. And this does not even cover the economic benefits of companies refraining from anti-competitive behavior.
There are therefore many benefits of searching for and prosecuting antitrust violations. It prevents other antitrust violations and the increased competition will benefit consumers and other companies.