Core strengthening can help with back pain, but in this case it's different. Walking on soft or variable surfaces causes less impact pain to sensitive nerves. People with poor flexibility or damaged discs in their back feel more pain from walking on hard, flat surfaces. The quality of shoe support / insoles can help with this too. If you have back pain when you walk, you start to compensate for it with an uneven gait, turning your pelvis inward or outward or tightening your hips. Over time this will cause tight muscles that will pull your spine out of alignment and exacerbate pain. Uneven terrain will force a break in these habits and encourage mobility and stretching in tight hips, hams and back muscles. This can be improved off a trail by doing mobility exercises like 3d lunge matrix, kinetic hip flexor and hamstring stretches. I would add that while you can prevent most back pain by doing core strengthening, wearing supportive footwear and doing these kinds of flexibility/mobility practices, it is always better for your body to have variability in how it exerts itself, than doing the same exercises over and over. Hiking is great for this because the terrain and the way you tackle it changes a lot each time you hike.