Luiz was signed on deadline day for £34 million from PSG after the Blues failed in moves for a string of their primary targets.
His return comes two years after he was sold to the French champions for £50 million.
Conte said: “This transfer market was very difficult for us to reach our targets.
“But at the last minute, on the last day, there was this situation which was good for me and the club to improve the squad.
“For this reason, we bought David Luiz. We invested in David, and I’m very happy.
“We took a player with a great personality, good technique, likes to play. I like to play football and I like defenders who like to play football.
“We know that David can improve in defensive situations, but it’s good and I’ve very happy he stays here. He’s a good guy, a positive guy for the changing room. I’m very pleased for this.”
Conte then went on to say thathis new signing could yet still develop into a world class defender:
He said: “I think that every player, until the end of their career, can improve. Every player.
“Also when you are 32, 33, 36… you can always improve. I tell you this from my experience as a footballer.
“I think that David Luiz is 29, a fantastic age for him to become one of the best defenders in the world. I’m seeing great commitment from him. He wants to play for Chelsea.
“This is a great opportunity for him and for us, also to improve. I repeat: he has good characteristics to become a fantastic defender – one of the best in the world.”