Does this support KML Polygon placemarks?

No - GPX only has waypoints and tracks, so only Point and LineString placemarks convert; a Polygon-only KML returns an error.

Why would I need this instead of just using Google Earth?

Handheld GPS units and outdoor apps (Garmin, hiking apps) typically take GPX, not KML - this bridges a Google Earth route into that format.

Convert a KML file to GPX for GPS devices and apps, in your browser.

Input (KML)

Output (GPX)

KML/GPX parsing only runs in the browser.

Runs entirely in your browser - nothing is sent to a server.

Does this support KML Polygon placemarks?: No - GPX only has waypoints and tracks, so only Point and LineString placemarks convert; a Polygon-only KML returns an error.
Why would I need this instead of just using Google Earth?: Handheld GPS units and outdoor apps (Garmin, hiking apps) typically take GPX, not KML - this bridges a Google Earth route into that format.