Why? Because they will change your hiking life. They are one of my most important pieces of gear. They are not just for those older bad asses that are still going strong that are finishing a 14er just as you're getting started. I will tell you why I love trekking poles.
They make your arms do some of the work and save your legs a little. When I use my trekking poles, I'm getting my whole body involved in the work out. That way, at the end of a long day of trekking, my legs aren't toast.
I don't know about you, but I sure as hell don't work this hard just to look at the trail in front of me all day. I can't tell you how many twisted ankles and harrowing falls have been saved by having three points of contact at all times. Having trekking poles allows me the freedom and comfort (once you're used to them) of being able to look up and at those mountains!
The first pair I ever got were from Walmart. They lasted a little more than a year. Not bad. The next pair I got from Costco. They lasted another year. The common factor? They both had the twist to tighten for adjustment. Eventually they always wore out and they wouldn't stay tight enough to use.
I finally went to Big 5 and got a pair that has the flip locks (Cascade Mountain Tech carbon). They've lasted over three years now. If I were to change anything about them, I would get the cork handles which are available as well. They work better for sweaty hands on a hot day. The rubber grips I have do wear off on your hands and is hard to get that off in the backcountry. But overall, I'm very happy with them! Sure, you can spend hundreds on a pair but these work just fine and allow room in my budget for something else. Someday I'll try those fancy ones and I'm sure I'll wonder why I waited.