Why is Grendels lineage given?

Answer 1

It's up to interpretation, of course, but Grendel's lineage links him back to Cain, as in Cain and Abel from the Genesis story. (For a quick reminder: Cain was jealous and slew his brother, and God cursed him.) This connection makes Grendel some sort of relation to man, in a twisted, dark way.

Now, Grendel becomes either a dark mirror of Beowulf--he has great power and strength, like Beowulf, but it's from the dark side of humanity and he uses his strength for evil--or merely a monster. Of course, he's never fully described beyond big with giant claw/finger/things, so he could be a man. Grendel's an exceptionally complicated character, which is to say that "why" questions very rarely have one answer and should, in any case, be taken with a grain of salt.