Sometimes it's innocent, but it's mostly an activity associated with malicious cracking....these people are seeing if they can get into business networks where the network admin was sloppy and didn't properly secure the WiFi connection.
With the proliferation of WiFi in the home sector, people are starting to use these types of antennas to steal internet connections from their neighbors. I can usually see three WiFi routers in my apartment building with the stock D-Link antennas on my server and my gaming machine although, my laptop card is only strong enough to see one of them.
If I wanted to get out of my monthly DSL bill, I'd build one of these antennas and piggy-back on someone else's connection.