I have a couple of quilts that were made in the 1930s that have the fabric brought from the back to the front. They both have the edge worn away. It starts to ravel away, thread by thread. So, the way I explain the reason I use bias is that when you fold the fabric from the back there is really one thread that runs all the way along that edge. It's the grain of the fabric. That edge gets more worn as the years go on..... edges always get the most wear. So, when that one thread wears out, the fabric rips. When you use bias, there are hundreds of threads, all crisscrossed along the edge. If one thread is weak, the other threads hold up. There is still more wear on the edge but so many threads make it last longer.