Keil and Delitzsch Biblical Commentary on the Old Testament offers this rather obvious comment:
A strong growth of hair was a sign of great manly power, and so far a
proof of Absalom's beauty.
The point of this piece of information is significant in the story for recording that Absalom's hair was both abundant, thick and strong. It made a significant contribution to his good looks and general appeal.
Thus, the story plot line is set - the most handsome man in the country is also the most vain who is eventually destroyed by the very thing that made him great - his hair. 2 Sam 18:9 -
Now Absalom happened to meet David’s men. He was riding his mule, and
as the mule went under the thick branches of a large oak, Absalom’s
hair got caught in the tree. He was left hanging in midair, while the
mule he was riding kept on going.
Thus, Absalom was helpless and shortly thereafter was executed by Joab.
The whole story "works" because the main character's downfall is caused by the very thing that he used to make himself great - his handsome appearance and beautiful hair.
Barnes also observes the same:
would seem that the two things which his vain-glory boasted in, the
royal mule, and the magnificent head of hair by which he was caught in
the "oak" (rather, terebinth or turpentine tree), both contributed to
his untimely death.
Thus, the weight of the hair (200 shekels - possibly 2.5 kg or almost 6 pounds) is a great deal and would have given Absalom a very strong neck to endure a sudden hanging in a tree - enough to snap the neck of a lesser person.