I use them when backpacking in mountainous or rocky terrain, especially if carrying a heavy pack. In addition to simply providing a huge safety factor in the event that you mis-step or loose your balance, by using two of them you can actually transfer some of the load from your knees/ankles to your shoulders.
You can spend a lot on trekking poles, but, unless you are planning on doing a whole lot of heavy duty hiking, I'm not sure it's worth the added cost. I have a pair of Coleman trekking poles ($14.99 each) that I have used on three trips though the Guadalupe mountains and they show no signs of wearing out. They are quite comfortable and I'm sure I will get many more years of use out of them. The length is adjustable, and I've never had any problems with slipping of the adjustment mechanism.
Coleman - Trekking Pole -
If the link is broken, send me a PM and I'll get it to you.
You may want to check out poles at an outfitter before you settle on a purchase. I looked at poles at an REI and then found these at the Coleman Outlet in San Marcos.
JohnK