Achilles allowed Patroclus to wear his armor to beat back the approaching Trojan army. It wasn't just a case of using whatever was near, and having need. It was a way to bring fear to the Trojans, and bring spirit of the fight to the morale of the Greeks.
While Achilles was the paramount and manifest warrior of the field, he was not always able to be present. He was often struck with black moods. When Patroclus pursued the Trojan defeated, to the gates of Troy, he was doing more than Achilles, and he caused havoc and disorder.
But then he was struck by Euphorbus, and then speared by Troy's hero Hector, and the victorious ending of war now turned turmoil. The Greeks and Trojans then exploded in battlefield ardor, over the body, of the one who had been thought to be Achilles.
Achilles sorrow was so great, that he took many lives as he grieved. The Trojans gained only Achilles's ire. Clearing the battlefield, Achilles called forth a fire, and soon every knot of hair from Greek warrior heads were burned upon the Patroclus pyre.
The end of life called for celebration, but Hades was waiting for Achille's friend. And only when Achilles was visited by the ghost of Patroclus, did his friend's time on earth would ever end.
