Long relievers often enter in the first three innings of a game when the starting pitcher cannot continue, whether due to ineffective pitching, lack of endurance, rain delays, injury, or ejection. The hope is that the long reliever will be able to get the game under control, and hopefully his team's offense will be able to help get the team back into the game. The hope is also that the long reliever will pitch long enough to save other relievers in the bullpen from having to pitch.
Long relievers are usually players who used to be starters either in the major leagues or in the minors (and still can be a temporary starter if one of the normal starters is injured or otherwise unavailable), but whose teams believe they have better starters available. Sometimes a team's long reliever is a former starter who has lost his effectiveness, either through a decline in skills or a series of injuries. Occasionally, long relievers are inexperienced pitchers who may have the potential to become starters or setup pitchers after gaining major league experience.
The quality of long relievers can vary, but when the long reliever is known to be an ineffective former starter, he is often called the "mop up man" or "mop."
A secondary use of a long reliever is in the late extra innings of a tied game, once the team's other, generally more effective, relievers have already been used. While a long reliever is often a team's least effective pitcher, he is still often a far better choice in an extended game than resorting to one of the team's starting pitchers (which can spread chaos throughout a pitching rotation, as everyone's future schedule gets adjusted), or even worse, resorting to a position player on the mound. A long man generally enters the game somewhere between the 11th and 16th innings in this role, and can be expected to pitch 5 or more innings, before a team will be forced to resort to other options.
Occasionally during the season, a team may find itself with enough rest days to allow it to use a four-man rotation rather than the now standard five. In these situations, a team may choose to keep their "fifth" starter on the roster in the long reliever role. This happens particularly in the post-season, when the fifth starter is a better pitcher than the "regular" long-reliever, allowing the team to carry either an additional short reliever, or position player, in lieu of the regular long man.