Remember that little screen that shows up right before a movie plays? It says “This film has been modified from its original version…” Why is that important? Well, formatting makes the difference between a super stretched-out image on your TV and a nice, square fit where you can actually see what’s going on.
It’s what makes the aliens in Independence Day look just as scary on your TV at home as they did in the theater.
Your websites are the same way (or, at least, they should be). They need to be formatted for mobile platforms, large-scale displays (like TVs and giant monitors), tablets, desktops, etc. Otherwise, your customers will get lost in the transition…or lack thereof. But there are some potholes along the way that some companies get themselves stuck in if their website provider isn’t using RESS methods correctly.