Filler is problematic if there is really nothing going on during your filler, and it's just an excuse to pad out your pages. Remember, there are plenty of ways to get quantity and quality. Padding isn't one of them. This is how to avoid filler and padding in your story.
What is the problem with filler?Some filler eposisodes may be well pulled off successfully, but filler should be used with care.
It detracts from the actual contentWhen somebody reads a story, they want to read about what's going on inside it. This is something that a writer feels a bit hassled about. They may not be sure if the actual content will fit the whole way. If this happens, then the author may resort to using padding to further it. But instead of clinging on to the story, making sure to follow it, the padding just decides to fall off and come across something unrelated, like the main character having a coffee break for no reason whatsoever. Imagine if 1984 had used up all the paragraphs to just talk about Big Brother's eyes. Sure, the eyes may be unnerving at first, but at the end, you find that they just detract from the themes of harsh control and pseudo-communism.
It doesn't do anything in the novelWhen writing about a leaf blowing in the wind, what do you do? Write a sentence or two about it, or write paragraphs about it?
If the answer is paragraphs, you're padding out your story. Unless the story is about the leaf, these paragraphs serve no meaning. Yeah, sure, you may describe it pretty well, but just because something is described well, doesn't mean it's going to do anything.