On a recent dayhike I offered a friend one of my two poles. She didn't want to deprive me of a pole. She slipped on a short hill and got a gash in her palm, which we cleaned out quick and bandaged, and I gave her the pole - she said it helped her a lot. She hadn't gotten hiking shoes/boots yet and was going up and down loose dirt/gravel on rock surfaces and other uneven terrain. She's going to get a pair of poles now, just for dayhiking.
Me, I think two poles used the right way, even on the flat parts, makes a huge difference especially while carrying a pack. Poles help on ascents and descents, prop me up when working my way over a fallen tree in the trail or wading a creek that turned out to be swifter than it looked, and make dandy snake fender-offers, or dog diverters, or what-is-that-thing pokers. They can be tarp/tent poles or weapons of defense. And they take weight off your back/legs; try hiking with them in nordic fashion, then carry them a while. There's a perceptible difference. I had to work with them a while to understand how to use them, but once it worked, I "got it."