Series adapted from an ongoing manga sometimes air episodes that appear nowhere in the source. Those episodes solve a pacing problem created by two publications running at different speeds.
The adaptation consumes source faster than it is made
A weekly episode typically covers more pages than a weekly manga chapter contains, so an adaptation running continuously will eventually reach the end of the published story.
Overtaking the source is worse than any alternative. The production would have to invent major plot developments that the original author will later contradict.
Original material inserted along the way slows the burn rate, buying the source time to build a lead the adaptation can then follow.
Continuous broadcast created the constraint
Shows that ran year-round without breaks could not simply pause, because the broadcast slot was committed and losing it risked not getting it back.
Filler was therefore the cheapest way to keep a slot occupied while preserving the adaptation. The alternative was reruns, which perform worse.
Sponsorship arrangements reinforced this, since financing was tied to a continuous run rather than to a fixed number of adapted episodes.
Filler is written under real limits
Original episodes must leave the cast exactly where they started, since the next adapted episode continues from the source's position.
Characters cannot grow permanently, learn anything that matters, or suffer consequences. That constraint is why the episodes often feel inconsequential.
Writers work within it by producing self-contained stories, comedic detours or background exploration that adds texture without moving the plot.
The split-cour model largely replaced it
Modern productions more often run a defined number of episodes, stop, and return after a gap that lets the source material advance.
Waiting is now viable because streaming removed the penalty for leaving a broadcast slot. An audience returns to a service whether or not the show aired last week.
The result is fewer filler episodes and more seasons separated by long intervals, which is a different inconvenience rather than an unambiguous improvement.
Adapted-material pacing changed too
Some productions stretch source material instead of inventing new episodes, adding extended reaction shots, recaps and slowed sequences.
Viewers generally regard this as worse than honest filler, since it degrades the adaptation itself rather than sitting beside it.
Productions with a fixed episode count and a completed source avoid both problems entirely, which is one reason adaptations increasingly wait for a finished story before beginning.