The squad network is the primary network most people will find themselves listening to and transmitting on. It's primary function is to facilitate the smooth operation of the squad.
Because all squad members (if they have a radio atleast) are part of the squad network, it allows for good situational awereness for all squad members. This also means that all squad members are aware of the tasking of their team and that of the other teams, enabling an easy succession of command when needed.
The squad leader should be the primary transmitter on the squad radio network. Most communication between fireteams should flow over the squad network. This keeps everyone's situational awereness up to date, and reduces misunderstandings, as verbal communication in hectic situations can become increasingly hard to understand.
Other than the squad leader, the fireteam leaders in turn use the squad radio network to communicate with the squad leaderm. They also occasionally communicate with their buddy teams over the radio in situations like bounding maneuvers, where shouting might not reach far enough in loud environments.