The plugin works in the Synaptop API, which gives you access to the functions described on the documentation page, as long as the project is being run as an app on Synaptop.
Here are some functions that I've found useful in the past:
Broadcast/Receive Message: This allows the app to communicate with other players invited to the app, thus allowing multiplayer functionality.
Display Message: This makes a pop up window with the desire message, this is useful for debugging and to notify the user of important events.
On New Subscriber/On Subscriber Disconnect: Helps keep track of what friends are currently playing the game with the main player, which is again useful for networking.
You can check out a few games that I have made for Synaptop with construct 2 on their website, you'll find them in the Synapstore, two of my games you might want to check out are Tic Tac Toe and Cards vs Mankind.