Anger leads to the dark side. If Luke falls to the dark side, then the battle outside the Death Star is pointless. We see Luke abandon the mission on Endor because he knows Vader is going to find him. He gives himself up and is taken to the Emperor and there tries to avoid giving in to the dark side. He has a moment when his anger takes over and I believe he taps into the dark side, but then he lets it go and is not consumed. He refuses to kill. The scene on the Death Star is a chess match between Palpatine and Luke. The fate of the galaxy rests in the result. If Palpatine successfully turns Luke, all is lost. And somehow Luke hopes to turn Vader, but his primary focus is to keep them occupied while the ground teams destroys the shield and then Lando and Wedge destroy the Death Star. At the point when Palpatine should be focused on the battle, he is instead distracted by Luke and isn’t paying attention to his impending demise. Had Luke fallen to the dark side, the Rebels would have been wiped out.
So Luke’s part in the final battle is not as a bystander, but a distraction. He picks up his lightsaber and distracts with the duel until Vader makes it serious by involving Leia. Palpatine is completely focused on Luke and trying to turn him. So Luke really is a hero for his actions because he prevents Palpatine and Vader from staying involved in the battle raging outside. Palpatine doesn’t even noticed when the shield generator is destroyed. He’d rather kill Luke and ends up being killed by Vader.
If I was Luke I would explain it by saying that Palpatine had been controlling Anakin/Vader all those years and that he was the catalist that Anakin needed to free himself. That puts all the blame for Vader’s actions on Palpatine. And I think that the way Lucas wrote the PT that is what was actually going on. I think Palpatine used the force to tip Anakin to the dark side and then used the force voice on him to make him destroy the Jedi temple. Then after the duel, Palpatine used the force to keep him alive all those years. So when Vader killed Palpatine he killed himself… not just because of the damage from the force lightning.
We know that Anakin wasn’t quite so innocent and that he was teetering and about to fall, but Palpatine did not leave things to chance. Watch that scene in ROTS after Mace is dead and listen to sound effects added to Palpatine’s voice. He is not leaving it to chance but is using the dark side to manipulate Anakin and hasten his fall to the dark side. I think Anakin’s anger at Obi-wan is genuine and that he really did try to kill Padme. And in the end he thinks he did. So when the chance comes to save Luke - Padme’s son - he doesn’t hesitate or think of the cost. What caused him to fall is what ultimately causes his redemption. Luke was the cause of the fall and redemption both. With Leia, of course, but Luke was the child he knew about leading up to that final confrontation. Still, Luke could follow Obi-wan’s example and tell the story from a certain point of view. Not like there are any other witnesses to the events.
But without Luke being where he was, Palpatine’s attention would have been on the battle and he would have sensed what was going on on Endor and could have given orders to prevent the success of the Rebel’s mission. But because he was distracted by his Sith business, he ignored the battle and was destroyed. Luke didn’t destroy the shield or the Death Star, but he facilitated both by being where he was and doing what he was doing.