This isn't a question about a specific anime. However, I wonder why filler episodes exist.
At first, I thought it was to allow the manga to advance a bit more when the anime was catching up. However, in Naruto (for example), the manga has finished and the anime still has fillers.
For other series like One Piece, the anime is relatively close to the manga and there are no fillers (Thanks God!)
What's the purpose of filler episodes?