And it's not just for websites.

Analogy: the doorframe is of a standard width so that a wheelchair can get through. This means manufacturers can make equipment that will fit in your house without calling you first! You never have to worry you bought a new refrigerator that needs to be pulled into a hole in the roof because it won't fit through the door. You never have to worry about delivering food to a grocer because your hand truck and boxes will fit, and there is a ramp instead of oddly shaped stairs. Nobody needs to be in a wheelchair for this.