At some point we were going to need things from Obi-Wan's point of view. Luke's pissed off at Ben for lying to him about Vader, but Ben turns the tables on Luke. For about a year Luke has been waiting for this conversation. We now know that he'd gone and found Jocasta Nu's stash of Jedi Lore, including a whole bunch of holocrons. He abandoned his training with Yoda for a time and decided to figure things out on his own. His return to Dagobah is him finally realizing that he can't hold onto his bitterness forever. Yoda's death and Obi-Wan's lecture on how people's perspective means everything is not what Luke was looking for, but it was what he needed.
Obi-Wan's explanation is his character capstone. The prequels were told through Anakin's point of view, tonally and plot-wise. We never actually got Ben's point of view at any point in the first three episodes and this is his only real chance to explain what he saw during that time. And... it's a doozy. Ben's explanation of the prequels doesn't line up with Anakin's in the slightest, and that's the point. The question is raised: does Ben have his head where the sun doesn't shine or is Anakin wrong to see what he saw?
That's a good question. I don't know. Nor will I try to figure it out.
Luke goes back to the Alliance, armed with the knowledge of who his sister is... and then promptly loses her. Huh, looks like we're still on Episode IV's plot. But, this time it's flipped: Leia finds the Ewoks. But their advent doesn't seems to phase Luke, who leans back into the Force and realizes that more's going on than meets the eye. And this is where 3PO's purpose is finally revealed. Throughout the whole series 3PO has been sort of hanging out in the background as comedic relief. He's gone through his own arcs throughout both trilogies and here, finally, the creations of Anakin finally bring him down thanks to his son, Luke. Even when someone has embraced the evil inside themselves there's no telling what good he has done may do that may come to help him. So don't discount what you've done, it very well may help you in unexpected ways one day.
This conversation is the fulfillment of so much in Star Wars. So much of the Prequels happened because people weren't honest with each other. Whether it be Anakin and Obi-Wan or Anakin and Padme, the whole plot of the prequels kept devolving as people put up more and more barriers up, between each other. The beginning of the end for this trend began in Episode IV with Ben's honest love for Luke. It continues through Episode V with Leia finally being honest with Han and Vader's honesty with Luke, and is fulfilled here with this conversation where Luke finally lets his guard down and reveals the truth that he's withheld from Leia. Remember, Luke has been absent in some manner for over a year, avoiding his friends, trying to keep the truth from them. But can't do it anymore. He's finally letting his guard down. Step by step Luke is undoing all the turn to the dark the he did between V and VI. Unlike Anakin, who hid from his issues and refused to air them to his friends, thus creating his own downfall, Luke is relying upon the friendships that he has in order to help them understand what he's going through and why his recent behavior has been so uncharacteristic. And Luke is honest enough to admit that he may be wrong. "I have to try" is not a confident "I will turn him", but "I very well may be wrong and I may have to kill our father and, while I'm prepared to do it, I have to try to save him first."
Sometimes people see something in us that we don't see ourselves. And, for all that we may protest, they might actually be right.