TCP/IP application layer protocols provide services for the computer software. The application layer defines the services that the application may need, NOT the application itself. For example. The application protocol Hyper Text Transfer Protocol defines how web browsers can pull web contents from a web server. Put it simply, the application layer provides interface between software on computer and the network it wants to be a part of.
The most popular TCP/IP application today is the web browser. Its just the most commonly used software, that so happens to use the internet as its primary function.