"Google Earth" is a stock of map obtaining all its maps from the internet. The advantage is that "Google Earth" can a huge of maps and can steadily up-to-date. To view terrain and roads (maps, aereal photos, satellite fotos) one needs an internet connection. At home it is no problem.
When you go outdoor without internet ,"Google Earth" cannot load new maps, but the maps which have been visited former have been stored in the computer. "Google Earth" remembers images and maps once shown in the screen in its so called Cache memory. This cache is stored to the local disk and therefore also available without internet connection.Therefore it is also called offline memory. Cache enables you to use Google Earth without Internet.
When the cache is full it overwrites the oldest data by newer ones. Nobody knows when the cache is full and less which data are automatically overwritten.
If I need a specific area after weeks for a presentation without internet these data are lost. There's only a fuzzy background without details. "Google Earth" has overwitten exactly this area cause someone has prepared his holiday trip on the same computer.
All the worse if you wanted to use "Google Earth" with GPS out of home and the necessary maps have disapeared.Google Earth itself cann't backup its cache data or to transfer it to another computer.
At this point apply "Cache for Google Earth"!