The boar was a very important symbol in Anglo-Saxon times, and it was used to refer to the Norse goddess Freyr. These animals were esteemed in Anglo-Saxon times for their cunning and ferocious natures, and even after Christianity was accepted by the majority of Anglo-Saxons, the image of the boar was still widely used in order to refer to these qualities. Note how the author of Beowulf describes the helmet that his eponymous hero wears:

...wonderfully formed, beset with swine-forms so that it
then no blade nor battle-swords to bite were able...

The boars herefore, perhaps because they hark back to a more distant and primeval system of beliefs, have the power to imbue Beowulf's helmet with protection. As a powerful symbol, the boars stand for all the qualities that were prized in warriors and are used to suggest the superior nature of Beowulf and his men.