GPX and KML are the two map storage formats accommodated by the plugin. Both are XML based formats.
The GPX format is mainly associated with Garmin GPS devices and has limited “data carrying.”
KML Keyhole Markup Language is associated with Google Earth (Keyhole) and largely recognized by Google products that care about such things.
The plugin likes the KML format because of its ability to handle large amounts of descriptive data. But in no sense is the plugin robust in terms of either format. It does what what it needs to do to meet its feature goals. No more. While the output of the plugin may work in other situations there is no intent to make that a goal. The maps are intended to be planning tools and not device navigation sources.
Handling of GPX is necessary because that is what the ACA provides. My Google Maps will handle some KML files from your own Google Drive.
Other map services like Ride With GPS (RWGPS) handle pretty much any map format.