Every now and then teams will see something that they like in a player that other teams miss. They then either trade for these players or sign them to low-risk contracts in the hopes that they can reach their potential.
These acquisitions often turn out to be decent ones, but occasionally teams get a steal. Finding a player that no one else valued as highly and having them produce at a high level is very rewarding.
Occasionally these players are just throw-ins to make a trade work and they end up being the best player in the deal.