You are supposed to upload only one (CSV or GPX). If you upload both then others report getting two POI files on their device.
The Garmin POI loader changes the CSV into a GPX. However the GPX files on this site contain extra information, such as distance away from the POI to trigger the proximity alert (i.e. select 100 m, 200 m, 100 ft, 200 ft... to trigger your warning for the POI).
The CSV format it nice to open easily in excel, though.