Filler episodes are important because they focus on character development and they give a break from the main plot.
I guess it depends on the show. Some shows are structured to work like that. Other shows like Game of Thrones, Black Sails, just aren’t.
Black Sails is a rare exception, but most shows would benefit from having filler episodes
I can’t see how GoT would, but then it depends what you consider as “filler”. I don’t think Dark would either. Or Silo
Dark would have definitely benefitted from it, I love it but it’s a drag to binge watch because it’s too much sad shit in a row