I used to use pillows between my knees when lying on my side and under my knees when lying on my back but while they did the job I did find it frustrating as when I moved around in the night suddenly all my support was gone and I had to reposition the pillows yet again. This tended to lead to a somewhat disturbed night!
With a full body pillow for support - well it is just blissful.... You are cradled and supported and comfortable...it helps to align the spine and hips and in some cases the neck as well so your whole body is supported, comfortable and settled. It takes the pressure off my back and allows me to r-e-l-a-x and certainly leads towards a much more restful sleep.
Yes it does take up room in the bed but that is a small issue and when you and your partner get a good nights sleep is soon forgotten!
There are quite a few designs and different kinds of body pillow supports. I like this one below but it is personal. There is nothing quite like a good, restful sleep.....