The TCP/IP communication module provides for a channel of communication for the SSL/TLS communication module to use. In the TCP/IP-model it provides for communication up to the Transport (or Host-to-host) layer. SSL/TLS resides on top of that, in the Application layer, and makes use of its basic provisions:

listening on a port (see mbedtls_net_bind()).
accepting a connection (through mbedtls_net_accept()).
read/write (through mbedtls_net_recv()/mbedtls_net_send()).
close a connection (through mbedtls_net_close()).

This way you have the means to, for example, implement and use an UDP or IPSec communication solution as a basis.

This module can be used at server- and clientside to provide a basic means of communication over the internet.