Angle and Cena had a feud last Summer when Angle was GM. They'll meet in the Finals of the tournament at No Way Out, and they had that No Mercy match in 2003.
Plenty of backstory.
But if Angle is retiring after this match that means he wants to retire now, not in August.
Of course, but I just said it would be better if it was the main event of SummerSlam, not some semi-main event of WrestleMania. Right now, it will be hyped as a Raw .VS. Smackdown match, not (possibly) Angle's last match.
But at the same time, if he gets a long reign as WWE Champion, and goes into Summerslam to defend against the top face(Cena), in a match billed as possibly his last match, everyone watching it knows he's going to lose, so that kills much of the suspense.
If Angle beats Cena in the tourny, and wins the title at NWO, before dropping it to Cena at SS (Probably in a classic match) where he retires, it would give Cena a much bigger rub than beating JBL at WM. They don't even have to advertise it as his last match... simply have Angle lose the title, and on SD he calls Cena to the ring, and makes a retirement speach.. talks about how he loves wrestling but his injuries are slowing him down and makes a speech about how he has respect for Cena after their match, and thinks he's the right man to fill his shoes, it would be HUGE for Cena.
Besides.. if Cena was champ and they advertised it as title vs. career, I'd expect Angle to take the title. If I didn't go on the internet, that is. I'd have no idea about the state of Angle's health, and him losing a match with his career on the line would come as a huge shock to me.