When doing duels in the horde or alliance valiants rings, a person riding a mount duels one of the npc valiants. Now usually these battles take a while because the npc's have 50,000 health. Now a normal knight has to break the shield wall and move around and stuff and its not a very easy quest.
Now when someone challenges the valiants someone not on a mount can kill the npc, DPS'ing them down to make the quest incredibly easy.
TLR Friend does valiant quest, challenges guy, you DPS down to make supa easy.