Because it was cool!
Other then it being against the Chiefs in "SB I" I really don't know.
Here is the Video. This is a rare item as there is no complete game film.
Why is this a historic play?
Then I have a follow-up to this.
Then there is this part of the tale as told by Mcgee
From the Sporting News
"I'd no earthly idea I'd play in that game," McGee wrote. "Neither Paul (roommate Paul Hornung) nor I expected to get off the bench. He hadn't played in about six weeks and I knew I wouldn't play unless Boyd Dowler got hurt. Vince put in a huge penalty that week, something like $5,000, and we all knew he meant it, so I didn't think anybody snuck out. The night before the game Dave Hanner checked the room at curfew and I asked Hawg if he was going to double check later. 'Yep,' he said. But then, as he started out of my room, he changed his mind for some reason. 'Nope,' Hawg said, 'I won't check your room later.' That was enough for me. I practically ran over him getting out of the room. I met some blonde the night before and I was on my way to pay my respects. I didn't feel I was letting the team down any, because I knew there wasn't a chance in hell I'd play.
"I waddled in about 7:30 in the morning and I could barely stand up for the kickoff. On the bench Paul kept needling me, 'What would you do if you had to play?' And I said, 'No way, there's no way I could make it.'"
"We sat together, discussing his wedding that was coming up, and suddenly I heard Lombardi yell, 'McGee.' I figured he'd found out about my sneaking out. I figured it was about to cost me $5,000. Then he shouted, 'Get in the game.' I almost fainted."
"Boyd was hurt and I played the rest of the game and caught seven passes and scored two touchdowns and after the game dear old Vince came up to me and said,
"Most any end could've done the same thing," I said.
"You're right," he said.
"I looked at him and said, 'Well, you sure took the edge off that, you s.o.b.'"