USGS Topo maps are only available at 1:24k, 1:100k, and 1:250k. When the satellite imagery map-pack is installed along with one or more state/regional map-packs the user gets additional zoom levels at smaller scales. The additional zoom levels are useful for a variety of things such as:
- Displaying a long route
- Panning cross state, country, etc
- Viewing tracks that are outside an area where you have map-packs.
The maps that Google Earth caches aren't compatible with Overland Navigator. Downloading Google maps for use in other applications is a violation of their license agreement.
Google Earth itself is a great desktop application, and one that I use frequently on my own desktop. In the field, I wanted to create a product that was easier to use. Bigger buttons, touchscreen/keyboard control friendly, simpler, and focused on in-field use-cases.