If I had to guess there might actually be no real tangible reason other than your cats might like the "mouth feel" of the piece of plastic. They might simply enjoy chewing on it. My poor old girl Ashleigh would sleep on top of my PC's tower even in the blazing heat of summer. She was kind of floofy too so I could never understand how she could tolerate the heat coming from the tower. Cats are cats and very individualistic at that.