Each season teams find themselves bringing sure guys up from the dregs of the minor leagues. Most of the time the move happens to provide that added depth in some positional regions. For the Cubs, a contending team and a team in first place, the latter is not true.
For the Cubs to bring someone up to the professional club, the team needs to make sure they will be a solid contributor.
That being said, at least in this instance, a majority of their talent is already on their Major League roster, so there may not be many names to consider for a modest comeback.