What is the advantage of the frame packs?
The frame keeps a lot of the weight off of your spine, instead transferring it to your hips. The rigid frame is what allows this, because it supports the weight. Then you have a padded hip belt that carries a lot of the weight. Of course, your shoulders -- and by extension your spine -- do carry some of it, but the frame reduces it quite a lot.
If you're carrying a lot of weight, you don't want it to rest on your spine. It's a bunch of little joints, and carrying weight requires more muscle use, and that results in back pain. By keeping the weight off your back as much as possible, you vastly reduce the amount of back pain from your equipment.
For a day hike, you needn't be concerned with having a frame pack for most conditions. If you're carrying a tent, sleeping bag, sleeping pad, tools to process fire wood, and enough food, water, and extra clothing for an overnight, you might want to look into a frame pack. I take about 20 pounds of equipment for an overnight, and some people take more; I can't imagine how my back would feel after a couple of miles with that on my back without a frame pack.