usually fillers are necessary in anime because anime are
afterall based on the manga.
usually the author will take 2 to 3 months (assuming at his fastest
pace) to come out with 1 book. but for anime, every week must come
up with an episode. so they must let the manga be many many
chapters ahead before they can do the anime.
secondly, the author will of course must prefer the fans to buy his
manga since he spent so much time and effort drawing them. so if
the anime follow very closely as the manga, everyone will just wait
to watch the anime instead of buying the manga.
lastly, most of time the filler storyline will suck big time cos
the author will keep the best ideas for his manga.
