The basic principle that responsive web design performs is versatility. In this day and age it's not just enough to get your website 'live', instead you have to also account for the variety of devices that users will be visiting your page from.
Android phones, iPhones, and tablets all offer different screen sizes, orientations, resolution, and layouts. Since these devices have basically overtaken laptops and desktops as the preferred method of web site browsing, it's important that your page is able to respond to any and all ways that people access it. Instead of creating a web page for each individual device, responsive web design ensures one page is compatible no matter which way visitors access it.