The unique thing about memory foam is that it softens in the areas where your body is in contact with it, forming a mold around those areas.
Many people use them as a leg spacer. Spacing your legs properly can relieve stress on your spine, align your hips properly and relieve the pressure of your knees being in contact with each other when sleeping on your side. If sleeping on your side with one leg bent, you can rest that leg onto the pillow which also takes stress off your low back.
They can also be used as a lower back reliever by placing one under your knees when lying on your back. We aren't meant to lie flat and placing a body pillow under your knees gives the knees a slight bend, relieving tension in the small of your back.
Doing this is much like sleeping on your back on an electric
adjustable bed with the legs slightly raised.
Another use is for leaning back on while reading, snacking or watching TV in bed. You don't have the normal hassle of having to stack several pillows in order to create a good cushion. One back pillow can do it all for you. When you have to lean forward, your pillow will stay in place better than a whole stack of pillows, which always seem to come tumbling down on you.
Another use for them is as a relief from an injury.
My wife uses one of these to prop her shoulder up a bit when lying on her back. She had a "horse wreck" many years ago which, among other things, left her with some serious back, shoulder and neck pain.
Using the body pillow in this way helps her sleep better at night.
Many pregnant women find relief by resting their "tummy" onto a memory foam body pillow. It conforms to their body and cradles them for a welcome pressure relieving night's sleep.
Sometimes I will sit on a memory foam body pillow to relieve my sciatic nerve condition. I spend a lot of time sitting at the computer, like I'm doing right now, and it really takes a toll on my "bottom."
It's a great help to have the cushion of memory foam between me and my office chair.
Tempurpedic® makes a one that is 36" x 14" and sells for about $150.00.