My understanding is the root weave is a pattern designed to be hard on your monitor(a crt when it was designed). It is ugly as sin but that tight flip from black to white was intended to expose any weakness in the driving beam, ether from misconfiguration or components failing. Where another pattern may obscure the problem. I think it is also rough on lcd's where a misbehaving one really sparkles on the weave.

I am not sure why it was the default, I suspect it was to give you a chance to see how your monitor was behaving on a fresh install and you were expected to set the background to something else.. I still run the root weave on my desktop, it is obsd with their xenocera where it is still the default. but I also run a tiling window manager so only actually see the root window once in a blue moon.