Websites have changed dramatically within the last few decades. One of the biggest changes is the fact that you no longer need a desktop computer to view and interact with websites. Whether it be a phone, a tablet, an iPad, etc..., they all need to be able to interact with the website in a design and interactiveness that is best suited for the device.
Responsive layouts for smaller devices can't just be a desktop view of a website shrunk down, they need to be redesigned and relayered around those devices. A good website will always have multiple layouts in response to different device screen sizes.
"Responsive Web Design always plays an important role whenever going to promote your website." - (Source: Josh Wilson)
These days a website is accessed more from a phone rather than a desktop computer. This means that even if the website looks and interacts great when viewing it from a desktop computer it does not guarantee that you'll get a new client or potential sale unless viewing the website from other devices such as a phone is an enjoyable experience as well.
