The purpose of using a WPA key is for security. This security prevents unauthorized access to your network. This protects your files and prevents someone from stealing your bandwidth. Survey tools such as Vistumbler, inSSIDER, NetSurveyor or Netstumbler can be used to see what type of protection you have, use GPS or Google maps and see the distance your wireless range covers.
WLAN Watcher (This software must be ran as an administrator in Vista and Windows 7) also keeps your network secure. After finding your WPA key and ensuring your network is secure, you can download WLAN Watcher, scan your network and mark all known computers. If any other computer appears, your network has been compromised and you should immediately change your WPA password.