Back Taxi?

I had a new tower instruction on a flight with a friend today. He was PIC and we had landed at RHV and were cleared for left closed traffic with the option from runway 31L. He flew the appropriate pattern and landed on 31L. As we slowed down and were about to turn off the runway RHV tower gave a clearance of back taxi 31L clear for takeoff 31L. Until this experience, back taxi is mainly used at non-towered airports that don’t have a taxiway.