star wars - Why did Darth Vader reveal to Luke that he is his father?

In my opinion, Anakin Skywalker and Darth Vader are different. In Return of the Jedi, Luke says, "I've accepted the truth that you were once Anakin Skywalker, my father". Then Vader says, "that name has no longer any meaning for me." But the question is: in The Empire Strikes Back, why does Vader reveal to Skywalker that he is his father?

Answer

To sway him to the Dark Side

Directly after informing Luke of his parentage, Vader turns off his lightsaber and attempts to persuade Luke to join him.

“Search your feelings,” Vader said, sounding like an evil version of Yoda, “you know it to be true.”

Then Vader turned off the blade of his lightsaber and extended a steady and inviting hand.

Bewildered and horror-stricken at Vader’s words, Luke shouted, “No! No!”

Vader continued persuasively. “Luke, you can destroy the Emperor. He has foreseen this. It is your destiny. Join me and together we can rule the galaxy as father and son. Come with me. It is the only way.”

_{The Empire Strikes Back (novelization)}

Presumably Vader felt that Luke would be more receptive to a peaceful approach from a family member than by threats of violence from the Emperor’s enforcer. This is borne out by Vader’s telling Luke that they can “rule as father and son.” Vader also knows that Luke never knew his biological parents. He loved the parents who raised him (his aunt and uncle), but Vader might well have suspected that knowing his biological father would have a certain appeal to Luke.

The revelation further serves to shake Luke's faith in Obi-Wan, and by extension everything he has been taught as a Jedi:

Luke’s mind whirled with those words. Everything was finally beginning to coalesce in his brain. Or was it? He wondered if Vader were telling him the truth—if the training of Yoda, the teaching of saintly old Ben, his own strivings for good and his abhorrence of evil, if everything he had fought for were no more than a lie.

_{The Empire Strikes Back (novelization)}

It also helps set up a feeling of inevitability. The son must join the Dark Side, as the father did. As Palpatine says:

Palpatine looked at Endor, then back at the lightsaber in his hand. ‘Ah, yes, a Jedi’s weapon. Much like your father’s.’ He faced Luke directly.

“By now you must know your father can never be turned from the dark side. So will it be with you.”

_{Return of the Jedi (novelization)}