Unpopular opinion: Filler episodes are good because most of them provide a nice break from heavy plot lines and it’s fun to see what the characters are like in different situations and sometimes they’re just plain entertaining
Good filler episodes reveal something about the world or the characters. And it doesn’t have to be something deep or heavy! A good filler episode can reveal something fun, something lighthearted. Something to just give the series more flavour.
Sometimes a filler episode can also introduce a concept, event, or character that will become important to the main plot later. It can also serve, as OP noted, as a break from heavy plots – and balancing your narrative mood is very important! It’s stressful for the story to be ON all the time, constantly at peak action or intensity. You have to rise and fall.
In my opinion, well-done filler episodes are not just good, they’re vital.