Misdirection, my friend. Get all the conspiracy wackos thinking they're looking at the real thing, when the actual Area 51 is on the outskirts of New London, Connecticut. You think a secret base would be that easy to find?
OK, I made up that bit about Connecticut. Or did I? *shifty eyes*
Google Earth is interesting; it blacks out entire sections of top secret locations. Area 51 IS said to be a bit of a decoy though. There's another base to the northeast of Area 51 called S-4 where it is said the crashed UFO was reverse-engineered. It's one of the theories behind how our technology has developed so rapidly over the past 70 years.