Not everyone is going to be a top prospect, or are even expected to make the majors. Some players are going to be used to fill out the minor league rosters, and can sometimes move up and down the system based on a need a level might have.
Some of the best baseball stories can come from a player who at first seemed like a career minor leaguer, only to put things together and finally make the majors.
What can make their jobs a little more difficult is that playing time is never a priority with them, so sometimes they can really get into a groove and even when they do it may not be enough to keep them in the lineup if the team has another player at the same position.