People don't access the Internet exclusively from their computers anymore: they are also using smart phones, tablets, smart TV's and video game consoles. New devices with different resolutions and technologies are launched frequently; creating and maintaining a website for each and every one of these devices is almost impossible.
This is where the "responsive web design" concept comes from: instead of making custom designs for each device, we can design one for an optimal experience that adjusts to the device that accesses it.