Besides aesthetics, the relief gives the skin somewhere to go when you bump into something.

When you run over a rock, stick or whatever, it pushes the skin inward. If you don't have a notch, as the object slides along it will hit the frame, pinning it between the object and the frame. This pinches the skin and could tear the skin. Most likely it will not tear it but I have seen it happen with a sharp object and a some speed.

Even minor bumps do some minor damage. If they happen repeatedly at the same spot the abrasion can eventually wear a hole in the skin. I have seen that happen in one day on a boat used for fishing on a flowing, shallow creek where there was lots of bottom bumping.

Jeff Horton
Kudzu Craft skin boats