Yes, pillows are supposed to be soft, fluffy, and comfortable, but they are also supposed to be functional. They're not just a place to rest your head after a day of screaming kids, phones ringing off the hook, or bosses breathing down your neck. Pillows are actually a postural device.
Yep, a postural device. Think about it. What other part of the day are you in essentially the same exact position for 5-9 hours? There isn't another time of the day. When you sleep, you are basically in 1-4 positions the whole night: on your back, on your right or left side, or on your stomach (a big No-No!). That means that not only do you need a good mattress with ample support, you also need a good pillow.
Now, it does matter what kind of pillow you should get depending on how you sleep. If you are a primarily a back sleeper, you need a "contour" pillow that has a roll to go under the curve of your neck. It also can not be too thick of a pillow. However, if you are primarily a side sleeper, you need a pillow with enough support to support your head. You want your neck to be in line with the rest of your spine, not to be slanted up or down the pillow. In both of these circumstances, the pillow will act as a postural device to keep your spine in correct alignment while you are dozing and dreaming.
If you noticed, I didn't recommend a type pf pillow for you stomach sleepers. And that's because you should NEVER sleep on your stomach. Even if it may feel good, it's not good for you. When you sleep on your stomach, your head needs to be cranked to one side in order to breathe (kind of essential). In this case, you are putting pounds upon pounds of unnecessary pressure on the bones, ligaments, tendons, and joints in your neck and upper back. And that can only lead to bad things down the road.
So if you take 2 things out of this, remember Support for your Side and Bend for your Back. Enough support to hold your head in alignment with you spine for you side sleepers and pillow with a bend or curve to fit the curve in your neck for you side sleepers. And of course, if you have any questions at all, feel free to comment or talk to one of the doctors in person.