Because a 20 minute anime episode can cover a month's worth of manga. If they didn't use filler they'd catch up to the manga faster than Game of Thrones catching up to ASOIAF!!Why is there literally 50 episodes of filler between 5-6 episodes of plot?
You're not missing much by reading the manga or just watching the show, but the benefit of reading the manga is you get to experience new content with a community, if you care about that.