Holding the ALT and click on any enemy's spell will make the system to warn the team "Beware of <spell name>".
In most critical situations (especially pub games), many players never use microphone to communicate.
If ALT + click works on ally's spell especially the ULTIMATE as well. It can be better for team to communicate.
Example; When blue player ALT + click on the Ultimate of yellow player = "Your <spell name> is needed" or any suggestion for how a spell call is needed/require by the team.