I imagine when you're knitting socks by hand- (or with whatever medieval sock loom may have been available?) it's going to take you almost as long to just make a plain white one. But after the first about two of those you get really bored. Even more so if you were making a full floor size rug. So maybe the motivation for the 'unnecessary design' on everything back then was less about the customer's vanity than the artisan's sanity.