Every baseball player goes through a cycle as long as they have the right amount of talent. They get adjusted to the game, hit their peak years and experience a decline. Such is true in all areas of life.
Every team goes through this cyclical process as well, having their peak years before needing to rebuild, particularly if holdovers are aging and need to be replaced. If a team is a contender and has an aging part, it's all the more important to have some insurance, such as a backup reliever or reliable fourth outfielder.
The following is a veteran from each team that either needs to be replaced outright or needs to have a backup added to the roster, just in case they falter next year.