There's filler, backstory and recap. Filler is typically not necessary to the storyline, but may be used as a building block. I usually watch, especially if the show is good.
Backstory often gets marked as filler, but people don't have the patience or foresight for the character development/insight to a character it provides. I watch.
Then there's recap...I usually skip (or let it play in scroll or somethin). Especially if I'm binging. Ik what happened I just watched it 🙃😂.