Virtual caches were introduced at Geocaching.com in response to the banning of physical caches in places such as National Parks. Since no container is required, a virtual cache could be placed anywhere, especially in places where a container would not be possible or advisable.
Visitors to virtual caches are often required to prove that they found the location by posting a photograph taken at the location, or by answering a question about the location. For example, a historical site might require the finder to locate a plaque at the posted coordinates and send email to the cache owner with a piece of obscure information found on the plaque. Photo requirements usually specify that you and/or your GPSr must be in the photo taken at the cache location. This assures that you actually were at the site and are not using an old vacation picture or a photo found on the internet.
Although many locations are interesting, a virtual cache should be out of the ordinary enough to warrant logging a visit. A common guideline was that a loction should be something "worthy of inclusion in a coffee table book."
In 2005, Geocaching.com stopped registering new virtual caches (including Earthcaches), although existing virtual caches were grandfathered. New virtual caches had to be registered as waymarks at Waymarking.com. However, in 2006, Geocaching.com started registering new Earthcaches again (but no other new virtual caches).