Pings are very useful for your allies.

You can quickly notify your team of urgent or important events/locations. Your allies will see and hear a noticable ping on their map and mini-map.

Just hold down Q and click your middle mouse button to ping your entire team. Your allies can then easily hit F3 to go to the latest Ping.

You can also add Labels, which are pings with text.
You can double tap Q and type your text for the ping, or double click with your left mouse, while holding down Q.