Why is the story of Patroclus so significant in the narrative?

Patroclus disguises himself in Achilles’ armour in order to scare the Trojans into retreating behind their walls. Patroclus basically sacrifices himself to give the Greek army some respite. Achilles is distraught at the death of his friend and whispers once he avenges his friend by killing Hector, he will join Patroclus in the afterlife.