Here at Flying Like The Pros, we use Flight Plan 1 as a backup to the active flight plan, which gets cleared whenever the Garmin avionics are powered off.
There are a number of circumstances where having a backup can be useful. For example, in the event you have to turn the Avionics Power switch off and lose the active flight plan, you are able to reload it immediately by activating Flight Plan 1. Similarly, if you receive a reroute from ATC and inadvertently make an error, you remain able to reload the original flight plan from Flight Plan 1.
Our most common use of Flight Plan 1 in many aircraft is to preload our IFR clearance into the GPS prior to engine start, as we show in the BAT 2 Clearance Delivery segment in the Cirrus SR22 Avidyne course. Since the avionics are usually off during the actual engine start, preloading the active flight plan doesn’t work. By preloading your clearance into Flight Plan 1, you will be ready to quickly and easily activate it after engine start.