The possibility to scan available channels without losing the current AP association. Since the Atheros card is always operating at a single frequency the driver can only optimize the time when to scan. If you are transferring data continuously, a scan will delay data packets. The driver behaves as if the device went into sleep mode, APs will queue data for a while.
The term "background" is quite misleading and many people thought that the hardware will somehow scan without having an effect on the transfer rate. This is certainly not true. In cases where scanning causes problems a second radio is needed (e.g. a second card just for scanning).
Usage: You can do
iwlist ath0 scan
wlanconfig ath0 list scan
in order to scan for instance for APs.
A complete scan requires a few seconds, during that time no data can be transferred. This is critical to certain applications if they require a reliable connection.