For creators, there might be a need to get the content out there to the consumers, and sometimes, in the storyline, to get to the climax may require a longer period of time. This is where they go creative, by instilling characters or scenes that weren't in the original source to achieve climax. Whatever they put in there is called fillers. This is what happened with the naruto filler list. The creators saw the massive followership, the series was gathering and saw the need to keep up with providing content. These contents weren't in the original setup, but they had to be given. The results? Massive. Hardly would you find anyone who doesn't know or haven't heard about Naruto, born in the 90s.
Take, for instance, in the original story, a battle can take a lot of time to end, but in filler, it might be shrunk to mere minutes and extended to the next episode. Do you know what this has done? It has made the viewers to long for more, to know when the fight is going to end, to know the victor and the victim. The naruto filler list contains this analogy. Most time, it is actually hard to stay a 100% committed to the source material. Difficult, especially if the series is gathering huge followership and reviews are pouring everywhere, there would need to tweak here and there. And what we have witnessed in some movies is, there are times when new creative directors are hired, those who the producers think are A-list. These creative directors would have to change a lot of things that might not necessarily fit into the initial storyline of the movie.