The Internet is designed to be an infinitely expandable network of computers. Therefore, servers and clients are only the end points of this network--there are additional nodes doing other tasks. For example, there are DNS servers, which map IP addresses to domain names; routers and switches forward the traffic. The web is one of the largest portions of the Internet, sharing specific, standardized content between end points. For creating a web application, the midpoints are out of concern. We only need to know how to configure web servers (backend), and how to write content for web clients (frontend).
There are a lot of other use cases of the Internet. Just think of video streaming, direct file sharing ...
