I like how the long, flexible shape supports the full length of my body. I can carve out a little enclave for my belly, rest my top arm on top, and wrap my legs around it or even stretch and straighten them completely while still keeping them fully supported.
If I want to switch sides during the night, all I have to do is keep my legs wrapped around it and just roll over on my back to the other side, and everything stays in place. With my old C-shaped pillow, this movement wasn't nearly as fluid. I'd have to throw my duvet off and and struggle to flip the cumbersome shape and size over my body before turning over and readjusting myself.
Image Source: Bbhugme