Large numbers of fillers in anime are less of concern due to modern anime season having a tighter structure. There are, however, several titles, especially classic and longer ones that have their fair share of them, to the point only filler guides can make them less intimidating. So, why do many anime have fillers?
What are Anime Fillers?
A filler in anime is an episode that doesn't advance the plot in any significant way. A lot of fillers can be safely skipped although, in some fantasy anime, fillers are used to introduce a power upgrade.
In the original Sailor Moon series, for instance, around 50 episodes in total are fillers with no real impact on the plot. At the same time, another 50 or so are associated with some update in the powers of the Sailor Senshi.
It's still possible to skip fillers of the latter category, though you might want to consider watching them if you are interested in a series' mechanics and worldbuilding.
Some action shojo and shonen anime fillers are concerned with a specific and plot-unimportant enemy with the conflict resolved within the same episode and then never picked up again.
In some cases, anime that don't normally have a lot of fillers might release one recap episode at some point mid-season, especially if they have gone on for a long time or if, for any reason, they cannot release new content at one particular week. This has happened with 86, Monster, and others.
Why Does Anime Have Fillers?
Many anime are created while their source manga is still ongoing. Fillers allow the show to keep going while they wait for the manga to catch up.
An attempt for that can be seen in Fullmetal Alchemist (2003) which, however, didn't prevent the show from eventually having to provide its own conclusion, as the manga wasn't completed until much later.
Of course, shows tend to have a tighter structure nowadays to fit into anime seasons, so most anime have 12 episodes, or 24 if they span over two seasons, after which they get renewed if and when new content is available.
Popular works with huge numbers of episodes might have not gotten away with their filler percentages if they were being made in the 2020s. This doesn't mean that all fillers are but though!
Fillers are an inconvenience for newcomers who are faced with hundreds of episodes and might not know which are the essential ones. When options were fewer and episodes were watched weekly on TV instead of online this might have been less of a problem.
Related: Anime Seasons Explained
It should be noted that even if an episode doesn't directly advance the plot it can still be fun and well-made. Plenty of fillers are worth watching if you have time and they can even better illuminate the characters and their relationships in a relaxed context, away from high plot stakes.
In the Sailor Moon remake, for instance, the lack of fillers was ironically commented upon, as some fans of the original felt that the new anime didn't allow enough time to get to know the main heroines and become attached to them.
Fillers or no fillers, it can be hard to find the perfect recipe to balance characterization, plot, and world-building. Fillers aren't the recommended solution for that but sometimes they can unexpectedly help!