I know that filler episodes are often done to keep a work going, or to flesh out a character's backstory.
In an anime that is its own work (not based on any manga, and with no manga counterpart), can filler be used as a means of taking a break from the story arc?
I know this is a different example, and a western work, but American TV drama series Grey's Anatomy was accused of having filler for much of Seasons 12, 13 and 14.
Filler isn't necessarily non-canon, is it?
Is it possible for a filler episode to later prove to be important even though it doesn't seem so at the time, if a character makes reference to the events of it further down the line, and how often as this happened?
I'm not an expert in anime, but is it more common for manga-to-anime adaptation than a series being entirely anime and created from scratch?
I'd appreciate any explanations, as I've seen varying discussions on filler.