For most people Google Earth was the first time they downloaded mapping software.
It was originally made by a company called Keyhole, hince the reason that files for it are called KML (Keyhole markup Language) files.
As of 2011 the wikipedia page claims it has been downloaded a billion times.
Unlike other mapping software it allows almost instant viewing of satellite images and datasets. This coming in the 2000s when it would take several minutes for ESRI to load simple shapefiles.
The imagery obtained on google earth comes from a variety of sources, most from the US government for free. The paid version allows you to go back easily in the archives to look at older aerial photography.
You can share data by emailing KML files and then converting them to other file types in more sophisticated GIS software.
Google Earth allowed users to add 3d buildings they drew using another software to have a 3d world. Rather revolutionary 10 years ago.
Whats amusing is Google at the request of various government censored various imagery of sensitive areas however those same aerial images could be easily obtained by downloading the imagery individually. The lone surviving gunman involved in the 2008 Mumbai attacks admitted to using Google Earth to familiarise himself with the locations of buildings used in the attacks.
In short Google Earth is a great distribution tool and very simple analysis tool in the GIS toolkit