Because the One Piece manga is still ongoing, Toei Animation uses non-manga material (filler) as a way to slow down the progression of the plot. If the anime were to catch up with the manga, Toei would either have to wait for Oda to release more manga chapters, take the story in a direction independent of the manga (as was done in Rurouni Kenshin , Elfen Lied , InuYasha and Fullmetal Alchemist ), or simply end it; any of these would likely damage the popularity of the series, and would not reflect well on either Toei Animation or Oda. Thus, additional material is the only viable alternative.
Let me tell you of a sighting of the Jersey Devil. I was driving up Route 9 in Bayville at around 10:00pm. There were two cars in front of me and we were traveling about 35 mph. To the right of Route 9 is a mini-mall type building with woods behind it. To the right is all woods. All of a sudden I saw this BIG thing running across Rt. 9! It looked like one of the classic pictures of the Jersey Devil. It had no tail, no fur, its ribs showed, and it had a long odd head with short ears that laid flat. It looked almost 10 feet tall! I noticed it because the first car stepped on its brakes, as did the second car. When I looked ahead I saw this thing galloping across Rt. 9 and straight into the woods. I was not really scared because it did not register yet. I stopped to mail something in the mailbox, about 300 feet from the main road, and I saw a child’s shirt, shorts and one sneaker lying on the ground! I mailed my letter, ran into my car and laid rubber all the way home!