WattMoodThey help with impact mostly. they also work to help prevent backward bends or "tacos". they do not do much for tailbone impacts or that kind of compression, and if they are worth it kinda depends on you. they can be uncomfortable and hot, but personally I choose to wear one because there is a lot of shit to hit your back on while skiing and I've had a few friends break their backs skiing. They have gotten much more comfortable over the past few years, with new materials and such. my original was a Dianese and it was very very hot. I recently got my hands on a Slytech backpack with a removable back guard and I can't really tell the difference between that and a normal pack, so for big/all mountain skiing I always wear it (but I almost always wear a backpack for shovel/probe/snacks when I ski so I'm used to having a pack on. if you ski without a pack it may feel odd).
The one I have prevents "tacos"! See above.