I have one designed for people with neck problems and couldn't believe how much more comfortable it is than the ordinary kind. The indentation in the center and thinner padding below my neck are exactly what I need to help keep my neck pain under control.
Other pillows are designed for people who have sleep apnea and must sleep connected to an IPAP breathing device.
Some are designed to give back support for people with spinal problems and may even provide vibration and heating to help relax tense muscles.
There are pillows for people who must sleep sitting up or in certain positions, too.
I'm grateful for my pillow and think lots of other people must feel the same way.