I agree with the idea they let you extend the groove across the back of the bow without violating the back ring. I not sure I agree with the idea the bow would fail otherwise, I've seen too many that didn't. I think it's important the bow not bend excessively too far on the outer limb though, and I don't care for grooves cut across the back without an overlay. They also let you achieve a more rounded, teardrop shape, again without violating the back ring. I've not had trouble with fast flight strings sawing through anything, on wood selfbows as soft as yew and elm, and on boo backed bows. I think they look good, cost nothing and if anything add a bit of insurance. More than anything string grooves should allow the string to lay straight to the back of the bow at brace height, and have a rounded teardrop shape to spread the compression load over as much material as practical, overlay or not.