Oh I love fillers!! :D There's so many ways you can use them, haha
You could use them potential
oohh, you thought it was a filler but it was actually deep-rooted foreshadowing >:) surprises or as breathing space for some comedic/heartwarming scenes, or ways to explore a side of a character to make them more human (and usually more likeable thereafter), or to introduce a new character, or to set up a new recurring series of jokes, or to just write what you always wanted to write, or to just wing it for your own satisfaction. (Sometimes the best fillers come from winging it, and sometimes the best ideas come from winging a filler.) And you know what? You could use them to fulfill
three purposes that I've listed here, and that's honestly the best feeling.
Good luck with your fillers, and have fun with them!
Edit: I just read all the previous responses and now I'm kind of sad that fillers are seen as a bad thing..... I usually consider them capable of being what really brings the reader into a story, giving a story its flavor and fleshing out the world and the characters, the bits that I always return to and remember after the fact because it made me laugh or endeared its characters/world to me more. ._. ah well, I guess