PC or gps download

Hi, I have a garmin e-trex 30, basecamp and mapsource, and am insterested in West Europe Map.
I don’t understand why there is one download for pc (basecamp) and one for the GPS. From basecamp, I can copy map to GPS, no?
What is the interest of downloading the one for GPS?

In case you don’t need (or want) the download for basecamp.

Just try to transfer W. Europe with Basecamp to your device. That will take ages.
If you have a fast internet connection, downloading and copying the gps img with your filemanager to your device will be much faster.