What you can embed: maps (normal view, satellite, hybrid mode), local search results, driving directions and personalized maps. You can't embed: street view imagery, information about traffic and mapplets.
This feature is useful if you want to place a map with driving directions next to the address of your business or to showcase some interesting places using a KML file. For example, the map below shows the most important colleges and universities from the US. If you click on the placemark, you can see the name of the institution. The map was generated from a KML file found using Google Maps.