Ever notice how some sites look horrible on your mobile device, but look just fine on a computer screen? This isn't a problem with your device, but rather a problem with the design of the site. Whoever designed it didn't have mobile devices in mind when they first designed their website, and now when you're looking at it on your screen it's squished up, or far too big which forces you to move the page around a whole bunch or zoom in a ton to see what's actually going on! But there's an easy fix for that, and that fix is RWD (Responsive Web Design)
Responsive Web Design is pretty much the future of web design. A few years ago the standard "It works on a computer screen just fine" mentality would have been okay, but these days more and more people are using all sorts of devices to view websites, so it's important that thoses websites look right when viewed from said devices. As technology improves, so too must web design. After all, what's the point of creating a site that looks great if it doesn't look that way on all devices?