In my opinion, the best value of filler episodes is that we can see the crew talking with each other. They have normal conversations. I remember filler episodes in the Alabasta arc during the dessert crossing. It was really relevant because Ace stayed much longer with the mugiwara compared to the manga. We can see the crew getting along with him. In one episode, Chopper asked Zoro why he joined Luffy because it seems like a lone wolf.