Why does Hrothgar ask Beowulf to battle Grendel's mother?

Grendel's mother is still grieving her dead son so she is pretty pissed off to say the least. Repeatedly attacking Herot sees to make her feel better. She also takes Grendel's trophy arm and Hrothgar's friend into her underground swamp lair. Hrothgar knows that Beowulf is the only guy who could possibly take on Grendel's mom. Everybody else is basically a scardy cat. He offers him a bunch of gold and treasure. Beowulf ends up chopping of her head. He then chops off the head of dead Grendel and brings it back to collect his reward.