NEED depends on your age and knees. I really need trekking poles now that I am over 60, mainly for long steep downhill (such as descending 5000 feet in a day- think Grand Canyon). Also for long trips (I did a 39-day trip this summer) I was able to keep going after I lightly sprained an ankle. Trekking poles have also saved the day during stream crossings.

At any age, properly used trekking poles can make you a faster and more agile hiker, particularly off trail. The key is "properly used". Good poles are made so you can collapse them and put on your pack when you do not want them (such as 3rd class scrambling where you need to use your hands). If you decide to use poles, get good, light weight poles and get some instruction on how to use them. I believe REI gives classes. I was a hold-out for years because I did not want to carry the extra weight, but once I learned to use them properly, I find them a real bonus. A nice side effect is that my arms get a work-out too and my hands/fingers no longer swell up due to carrying a pack.