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Hrothgar is the king of the Spear-Danes when Beowulf arrives in their land, ready to fight the demon Grendel.
We're going to let that sentence sink in while you marvel at how much cooler names were way back when.
Many years earlier, Hrothgar paid money to the Wulfings to resolve a blood feud they had with Ecgtheow, Beowulf's father. (Again: these names!) As a result, Beowulf feels some loyalty toward Hrothgar.
Although Hrothgar was a mighty warrior in his day, at the time when the epic story in Beowulf takes place he is an old king, no longer able to defend his people against the marauding demon Grendel. He'd probably rather be playing shuffleboard at this point. But he's also without a ready successor: his two sons, Hrethric and Hrothmund, are both still too young to take his place. King Hrothgar therefore represents a serious problem for medieval Scandinavian tribes of warriors: the dire threat posed to an entire tribe by a king who has become too weak, or by any kind of power vacuum.
Luckily for Hrothgar, he has a savior in the form of Beowulf, who is prepared to aid him by defeating the demons that stalk his land, but is too principled to steal his throne. Of course, even though Hrothgar lacks the strength as a warrior that would be necessary to bring true security to his people, he practices another custom that does help maintain his power—ring-giving. Hrothgar generously distributes rewards, including gold, armor, rings, and horses, to the warriors who support him, both Dane and Geat:
Then Halfdane's son presented Beowulf with a gold standard as a victory gift, an embroidered banner; also breast-mail and a helmet; and a sword carried high, that was both precious object and token of honour. So Beowulf drank his drink, at ease; it was hardly a shame to be showered with such gifts in front of the hall-troops. (1019-1026)
As a result, he can literally buy loyalty, even when he can't force it in battle.