When the fabric is just folded under and sewn to the tab, all the stress on the awning is on the stitching alone. A failure of one stitch puts much more stress on the adjacent stitches, increasing their chance of failure. If the stitching fails, there is nothing else holding the awning on to the channel.
If the fabric goes up and over the awning rope, even if the stitching fails, the fabric going over the awning doesn't necessarily move, so the adjacent stitches don't get stressed.
Long story short, it's more stable and less likely to lead to the canvas separating from the trailer.
Sailrite had a video of Sunbrella being sewn just to the tab. If I were doing something that just needed to be held in place but wasn't under a lot of tension, like weather cloths on a sailboat, I'd probably go that way. The plain plastic awning rope slides more easily than when the cloth is over it. But, for the same reason, I like the cloth over the awning rope on my Airstream. I don't want the rope to slide easily out of the channel.