What is the purpose of the Sigemond and Heremod digressions?

The poet uses Sigemond and his killing of the dragon as a positive comparison with Beowulf even though the results of their respective fights are vastly different—Sigemond kills his dragon and takes the dragon's hoard whereas Beowulf kills the dragon and saves his kingdon, but dies in the process. The main point of the Heremod digression, however, is to point up the difference between a bad king, who fell victim to "floods of sorrow" and betrayed his duties as king, and Beowulf, who has already shown his ability to protect his people and his stronghold (Heorot). A secondary but perhaps equally important reason for the digression is to discuss kingship. Even though Beowulf's fight with Grendel appears as the centerpiece of the poem—the action that readers remember—it is just one episode in the long life of a king who ultimately is known and loved for having created a powerful and peaceful kingdom for the Geats and who dies protecting his people from evil (the dragon).
https://www.owleyes.org/text/beowulf/read/xiii