My first experience with poles was doing the Coledale Round in the Lakes. For anyone who does not know it, there are a lot of ups and downs. I had no poles and was ok until about two thirds of the way round when I started getting knee pains. A friend in our party offered me his poles to try. I was so impressed, I bought some as soon as I could and have used them on steep downhills ever since.
As I have aged, I have found they avoid back ache on steep ascents too.
As far as the downhill knees are concerned, logically, they are taking the combined weight of you and your pack. With lots of walking and age, it is the cartilages which start to go and distribution of the weight through poles, as long as you use them properly, saves some of this pressure on the joints, so reducing any pain. Lots of young Alpine guides use them to avoid problems in later life.