Unless they don't

Filler is anything that happens that didn't happen in the source martial
There is a difference between character development, world building, and random chaotic fun
Character developments if forgotten in the next episode then the character development is void.
World building is something that doesn't contribute to character development but add more depth to character interactions and the world they live in
Random chaotic fun is events that just happen for gags and don't realy contribute to the world or characters.