John Gardner chooses to retell Beowulf from the monster's point of view because he wants to highlight what he sees as the capacity for monstrousness that exists in each and every one of us.
Grendel may be a monster, in Gardner's account no less than in Beowulf, but he also has recognizably human characteristics. Because of this, he comes across, at various points, as engaging and broadly sympathetic.
Grendel freely admits to his destructiveness and the loneliness that drives him to commit atrocity upon atrocity at Heorot, the mead-hall of the Danes. This makes him seem like a tragic character rather than the one-dimensional beast we see in the original epic poem.
The humanization of Grendel in Gardner's novel forces us to question the stories we tell about ourselves, especially those, like the original Beowulf story, that unthinkingly validate old notions of heroism while downplaying or even ignoring the monstrousness that lurks within even the noblest of heroes. Because of this, we come to see humanity the way that Grendel sees it, as flawed, occasionally ridiculous, and with a seemingly endless capacity for violence.
This heightened level of self-awareness that this shift in perspective brings is arguably the greatest gain to be had from Gardner's retelling of the old legend.