It was one of the best changes that Google Earth implemented in the 5 version, which at the time of allowing us to see which year images were published makes it easier for us to use the best resolution or relevance for our purposes. In many cases, because the most recent image has clouds that hide the object of our interest and in other cases because the level of detail was better.
To see the history, the icon of the clock is activated, then you can drag the bar to go to the date of change of image. Although the most practical is with the arrows at the ends, which lead to the next, above is the date on which it was recorded (possibly the year of the shot), not necessarily having been uploaded to Google Earth.
To show this example, a project that I want to georeference.
This is the January image of 2010, see that the boundary of the polygon can not even be seen, despite the fact that the buildings at the top are already built and for cadastre purposes are more important because they imply a reappraisal of the improvements.
This one is from the 30 of November of 2007, 4 years before and see that clear the limit is seen. Above are not new buildings and the rest of the take is covered in annoying cloudiness. The only thing I can not solve, is that when downloading with Stitchmaps the bar of the historical appears annoying at every shot; One of my technicians made me the joke that we tell people that they are alien positions.
And the latter takes the preliminary draft of planned urbanization, safe in about four years can already be seen development.
In matters of precision... it is a calamity, because between one shot and another there are up to 14 meters of difference ... and none comes close to reality. But for purposes of impact, if there is profit from what Google Earth and Google Maps have achieved, it has made geolocation reach everyday use.