Are you referring to the Garmin Helm app?
I was shown this option when my boat was delivered - I'm using Android LG V20 phone. The WiFi connection and use of the app was pretty much straightforward, no issues at all.
Every screen and menu that is available to you on the Garmin chartplotter is fully functional on your device - but as with the Garmin GPS, you still don't have access to the throttle.
As for using it - I personally don't see a benefit (at least while I'm learning the new boat systems, etc - this may change over time) using the connection while underway mainly because I am sitting at the helm. I might see the benefit of using the WiFi to a tablet or phone once I'm in a slip, etc and simply wanting to control the radio or even look at the charts.
One thing I did notice - if you use your phone and stream Pandora (or any other music streaming service over LTE) and you then use Bluetooth to connect to your Fusion radio - when you setup your phone for WiFi to connect to Garmin, you loose your LTE connection - no more music on the radio. Probably best to keep one device connected to the radio and a separate device connected to Garmin.
