Those old 60s Batman fillers had him solving random crimes, but they showed his detective side you rarely see in big events. Skipping them makes the character feel thinner somehow.
Look, @jenniferhart has a point about character moments, but filler can mess with pacing. When I just read the big X-Men arcs, I see Cyclops shift from leader to outcast without all the side stories. Those extra issues often repeat the same ideas or slow down the main plot. In Spider-Man, skipping fillers lets me see Peter's big life changes one after another, which feels more powerful. It's not about missing things, it's about keeping the story moving forward without the fluff.
Tried reading Claremont's X-Men in order once and the fillers killed the momentum for me. Skipping to the big beats like Dark Phoenix made the character changes hit way harder.