Animation companies utilize fillers for a number of reasons:
** If the companies feel that the anime is catching up to the manga too quickly, they will insert entire filler episodes to allow the manga writer to write more chapters.
** Anime versions are usually able to adapt at least two manga chapters per anime episode. If the anime writers feel that a crucial chapter should not be aired in the last part of an episode, they will add a filler segment to make up for this, and air the crucial parts in a fresh episode.
** If the anime gains huge popularity amongst the viewers, anime companies often add fillers to extend the anime, capitalizing off the popularity (one example of this is Naruto, airing fillers for over a year, turning the anime stale).
<NARUTOluvr669> omg, i hope dey, lyk, stop showing doze NARUTO fillerz dey r soooooo ghey!
<SasukeChidoriSharingan666> omg lol i kno wut u mean!
Fred: Hey, I heard the Kenshin anime added fillers and never returned to the main storyline.
Gil: Ouch man.