Symbolism, Imagery, Allegory
Grendel's home life does nothing to improve his sense of self or help him understand his place in the world. What do we know about Grendel's relatives? Well, they don't care much for sunlight, spoken language, or personal hygiene. Mama has direct contact with her son and seems to show some kind of maternal instinct for him, but she still can't talk. In Grendel's book, that puts her right down there with the brute animals of the forest.
There are moments when Grendel thinks that Mama is communicating to him in a prophetic way—"My mother makes sounds. I strain my wits toward them, clench my mind. Beware the fish" (149)—and he can see that she worries about him getting in with the wrong crowd, but that's about as far as it goes.
As for those shapeless, distantly related figures with glowing eyes and stinky fur in the background... well, they just seem to grunt all day and stare. Grendel can't interact with them at all, and he doesn't even understand why they are there.
The cave-dwellers represent a kind of mystery about Grendel's heritage—a clue as to whether or not the Shaper is lying when he sings about Grendel's race being cursed by God. Grendel tries to get closer to the truth by confronting the miserable subterranean existence of these cave-dwellers. Who are they? If he could figure that out, he thinks he could figure out a lot about his own identity and his own purpose.
But since his mother can't communicate in any useful way, Grendel's left in the dark. He has to search for meaning in the meadhall and in the dragon's lair. In a way, he's like all of us: we don't really know why we're here, and the only answers we can find come from our interactions with other people and their ideas.