The best description that I have heard is what OK said, but different. If you have dial up it takes a long time to download info. A larger cable tranmits more info and so it happens quickly. If info were energy, the same theory applies. So if you're needing to cast large flies, you need to generate more energy, and the heavier the line weight the more energy it can carry and transfer. As for the fishing aspect I have a theory there too, but I like the internet allegory, so I thought I would pass that on.