The entire purpose of the lower three layers (Transport, Internetwork, and Link) is to make it so that applications running in the Application layer can focus the application problem that needs to be solved and leave virtually all of the complexity of moving data across a network to be handled by the lower layers of the network model.
Because this approach makes it so simple to build networked applications, we have seen a wide range of networked applications including web browsers, mail applications, networked video games, network-based telephony applications, and many others.
And what is even more exciting is that it is easy to experiment and build whole new types of networked applications to solve problems that have not yet been imagined