When you want KML
Google Earth drapes a KML line over 3D terrain, and Google My Maps imports it as a layer that shows up in the Google Maps app under your saved maps. Both also read GPX, so the conversion is only needed for tools that insist on KML. Opening a Strava route in Google Maps walks through both.
What the KML contains
A single placemark with the route as a line — longitude, latitude and elevation per point. KML lines have no timestamps and no sensor data; if you want the time and heart rate kept, save the GPX instead, or keep the original file.
In your browser
The file is parsed and rewritten on your machine and never sent anywhere. FIT and TCX files convert the same way — drop them as they are, zipped or not.