The battle robe top is a reward that can be purchased with tokens from the Fist of Guthixminigame. It degrades with use similar to the Barrows equipment and costs 150 FoG tokens to repair, however it may only be traded when completely degraded. It requires level 50 defence to wear.
Similar to all other armours obtained from the Fist of Guthix minigame, it can be degraded for trading purposes by talking to Reggie in the cave and asking him to degrade the item.
When used with a staff of light, and a full set of battle robes, there is 50% chance that you will not use any runes when casting a combat spell.
After a total of 6 hours of combat the robes will be degraded (from 100% to 0%). This won't be affected by how many runes the robes save - it's just the time spent in combat - so using the staff of light does not prolong their lifespan. This may affect the reason for buying the robes - be it on the Grand Exchange or from Reggie.