Why did the author of beowulf make such a point to describe the "heathen"rituals used to try and ward off grendel?

That could be the full poem, if it's written in verse. The purpose of this work, an epic poem is not just to tell a story but also to memorialize the culture that it relates. The rituals of a culture are one of its touchstones, so the singer/writer would want to emphasize those for an audience who would want to remember/appreciate those details. But the failure of those rituals also sets the way for the hero, Beowulf, to save the day through heroism.