One of the necessary aspects of team building is getting surplus value. Getting a player who outperforms their contract is the path to getting value. We have a tendency to think about this only through draft picks (and it’s certainly where the most value can be obtained, as draft-pick contracts pale in comparison to free-agent deals on average), but having free agents outperform their contracts can be a big part of building an effective team in a limited resource environment like the NFL.
Some free-agent contracts are given out to known quantities. The team ends up paying a premium for that known quantity and although that player may play up to their contract, they very rarely exceed it.