You may find a player released that is better than one of your late round selections or that currently reside on your reserve squad. This often happens in April when a player starts the season on the DL, get replaced by a good player, then gets activated. Watch the performance of free agents and select them to better your team. Many times a possible closer goes undrafted and is available as well. However, you have to hunt the box scores and research the web to stay one step ahead of the competition.
Every successful team takes time to keep the roster current and with the best available players.
Looking to have pitcher in the line-up that has two starts in the current week is one example. Knowing when your players go on the DL and replacing quickly is another. Sometimes knowing when not to replace a player, in the event that they could hurt your batting average or ERA, is important too. More often than you think, no replacement is better than a bad one if nothing decent is available. A DL'ed pitcher will never give you a bad inning of work.