For people like me who have a smart phone and don't have a service provider. VoIP is my mode of communication. By mapping out the open AP's in my area, I know were I can walk around and still retain something similar to cell service. I also am able to spot the AP's with weak protection (WEP, which can be broken in about a min). 
It's also just another intresting way to get out of the house and walk around. It gives you a real reason to go down that dead-end street that you would not walk down if you were just going from point A to point B.