Hrothgar, king of the Danes, built the mighty mead hall Herot for several reasons, both of which reflect the context in which the epic poem Beowulf was composed.

Hrothgar wanted to build the hall as a place for his warriors to gather. It was a convention of the times that a leader reward the efforts of his warriors with feasting and gifts--this was how they earned and developed loyalty, which, in an era of smaller competing kingdoms, was vitally important. 

But Herot was also important as a religious symbol. Denmark, like many areas of the world, especially Europe, was in a period of religious flux as the old mythologies gave way to Christianity. Burton Raffel's translation of Beowulf states:

Hrothgar . . . resolved
To build a hall that would hold his mighty
Band and reach higher toward Heaven than anything
That had ever been known to the sons of men.

From this we see that Hrothgar and the Danes intended for Herot to symbolize a spiritual yearning for God. We can infer that it is the Christian God they wish to reach by other statements in the work which equate pagan polytheism with the devil.

Hrothgar built Heorot (Herot, depending on translation) for the "best and bravest of his men." It is an honorable king who seeks to reward his soldiers for their bravery. The building was beautiful, both inside and out. Extravagant in decor (it was covered in hammered gold), the hall was supposed to be a tribute of Hrothgar's pride in his men, but Grendel, who could not stand the idea of happiness, made it a place of horror.