Yep. Some people put stuff in the "wrap" step too... I've heard of apple cider, additional rub, honey, etc. Usually they go on for the first 3 hours with the bones facing down, then during the wrap you put them meat-down, and back to bone-down for the final unwrapped step.
But the idea is that the first 3 hours give you the smoke flavor and a little bit of bark. The next two hours is effectively a braise to get them tender, and the final hour is to reset the bark and dry out the surface so it's not mushy. Also that final bit (20-30 minutes), if you sauce your ribs, is a good time to get some sauce on them and let it warm up and get sticky.
Baby back ribs are often suggested that you go from a 3-2-1 to a 2-2-1, but I find myself preferring 3-1.5-0.5 with baby backs... I rarely cook spares, actually, primarily baby back. Although I probably need to start increasing my times to maybe 3-2-0.5 because I usually buy really meaty racks from Costco that take a little longer.