Bus A/T
Never actaully done this myself, but have seen a few people use this button to "recylce" the system on approach when autothrust maybe slow to capture Vapp. IE Vapp (correct target approach speed) is plus 10-15kts and the thrust is still coming on, some guys press the A/T button to engage "thrust lock" ( basically locks the thrust at time of pressing this button) thus by resetting the system seems to give the autothrust a little kick into life to get the power to spool down and achieve Vapp....
Apart from that, no I've never seen it pushed for anything else under normal ops.
Hope that makes sence....
Cheers