The issue is if airports don’t have a parking spot to start cold and dark from, be it one on land or a dock spot in the water.
Asobo fixed the ability of amphibians to drive in and out of the water a long time ago, so Amphibians can start at any airport with a parking spot. For instance, I didn’t put any land runways in my 52B airport, but I put a couple of parking spots on land on ramps to the water for people who like to start cold and dark from land for their amphibians.
Float planes require a dock parking spot, but can start cold and dark from them as well.
If you’re trying to start up cold and dark at a location where an airport or SPB is not nearby, however, that tool is useful.
As a side note, many people wonder how pilots do run ups when they’re in the water. They wonder why the plane moves when they crank up the rpms. Happens in real life, too. We as engineers are still working on developing brakes for the water… soooo, they do their runups along the way when taxiing into position (it’s not a good idea to do a run up when tied up at the dock as this can overstrain the floats, though I have seen / read of instances where operators do that, too).