All this is EU crap. That line makes Vader seem weak and with no agency in the movie. It’s why Vader kinda sucks in ROTJ compared to ANH/ESB, at least until he brings Luke to the Emperor.
Yeah, it makes him feel like a real person underneath the suit. Let’s look at the script!
Vader and Luke are left standing alone in the oddly tranquil beauty of the place. The sounds of the forest filter in upon them.
VADER: The Emperor has been expecting you.
LUKE: I know, father.
VADER: So, you have accepted the truth.
LUKE: I’ve accepted the truth that you were once Anakin Skywalker, my father.
VADER (turning to face him): That name no longer has any meaning for me.LUKE: It is the name of your true self. You’ve only forgotten. I know there is good in you. The Emperor hasn’t driven it from you fully. That is why you couldn’t destroy me. That’s why you won’t bring me to your Emperor now.
Vader is pushing for Luke to join the Sith, because Lucas hasn’t invented the Rule of Two yet. Luke is trying to remind Vader of his past as a good-guy, because he wants him to forsake the Empire and join the Rebellion. Vader doesn’t want to talk about that, so he changes the subject.
Vader looks down from Luke to the lightsaber in his own black- gloved hand. He seems to ponder Luke’s words.
VADER (indicating lightsaber): I see you have constructed a new lightsaber.
Vader ignites the lightsaber and holds it to examine its humming, brilliant blade.
VADER: Your skills are complete. Indeed, you are powerful, as the Emperor has foreseen.
He also takes a moment to talk-up his boss as an all-powerful (and thus irresistible) wizard. How can Luke say no to that?
They stand for a moment, the Vader extinguishes the lightsaber.
LUKE: Come with me.
VADER: Obi-Wan once thought as you do.Luke steps close to Vader, then stops. Vader is still.
Luke is pretty sure he’s getting through to him - Vader brought up Obi-Wan! His old friend! Remember the good old days when you were a good-guy?
VADER: You don’t know the power of the dark side. I must obey my master.
LUKE: I will not turn…and you’ll be forced to kill me.
VADER: If that is your destiny.
LUKE: Search your feelings, father. You can’t do this. I feel the conflict within you. Let go of your hate.
VADER: It is too late for me, son. The Emperor will show you the true nature of the Force. He is your master now.
Vader, however, wants Luke to understand that, as far as he’s ever seen, there is no other option. He’s very conflicted, but what can he do? The Emperor sees all, can do all, etc. Vader doesn’t want to die, and he doesn’t want Luke to die. The only way to not die is to obey the Emperor.
Vader signals to some distant stormtroopers. He and Luke stand staring at one another for a long moment.
LUKE: Then my father is truly dead.
Anyway, my point is that this line is an essential part of Vader’s journey. It shows that he’s no longer actually committed to the Empire or even the Emperor, he just sees no other option. He is in despair, without hope, particularly since he’s pretty sure his only child (that he knows of) is committing suicide by trying to defy the Emperor. It demonstrates that Luke is right about him, he just doesn’t see a way out.