why in one-take?

It's bragging rights for the director because it shows that it was done without any cuts. It's like the beginning of Boogie Nights or the scene in Goodfellas where Ray Liotta and Lorraine Bracco go into the Copa. In this film the director is a massive ego maniac so he wanted it so everyone would say how brilliant he is to pull off all those stunts in a single shot. In realty, I can't imagine a director would risk it. The opportunity for error is too great with all the gags that have to be perfectly timed. Too dangerous.

I don't think a sequence like that would have to be in one, unbroken shot. The idea here, however, is that the stunt work is so complex and dangerous, utilizing structures that can't simply be set up again for a retake (I think the character of Tony points this very thing out). Also, the entire scene is being shot by multiple cameras from the air and ground, necessitating massive coordination amongst cast and crew.

I don't think many modern viewers realize how difficult such scene could be to shoot in the pre-digital age. All the effects were practical and had to occur - more or less - in reality as they would appear on the film. Optical effects were rather limited to matte paintings (static backgrounds usually hand painted unto glass) and animation.

When filming "Jaws", for instance, Spielberg had to wait every time a single boat appeared in frame in the horizon, since he wanted to give the impression that his characters were isolated at sea without help of any kind. As you could imagine, it could take hours for such a boat to travel across the horizon and out of shot (today, a stray object like that could simply be digitally removed). He also had to coordinate several boats at once on the ocean, which seems simple but is incredibly complicated. The Orca, the camera boat and the equipment barge all had to be stationary in order for a shot to take place. With a rough tide, anchors drift all over. Throw on top of that all the needs of the actors, crew and special effects and you have what resulted in a nightmare for Spielberg.

Cause they are blowing up rral buildings like in the Dark Knight..and you cant rebuild them....dont over complicate something ...sometimes a cigar...is just a cigar.