[Jasoncw]

It's to have a more dynamic composition.

Modernism is more open to having offsets, pinwheels, layering, and other compositional techniques, rather than the old beaux arts architecture which tends to favor static monumental axises. This makes a bigger difference at ground level in the plaza, but even in skyline shots, most of the interesting photos of the WTC you see are only possible with offset towers.

This is also the reason why there are two towers instead of just one, because having two of them creates a pair of compositional elements which can be arranged with each other.