phase shift

Because 360 degrees divided by 3 (phases) is 120 degrees. This results in the largest possible potential between the three phases at all times.

Or were you asking something deeper?

its balance.

In a synchronous generator, the three windings or coils with terminals a-a', b-b', and c-c' are physically placed 120 apart around the stator. If the rotor magnetic field rotates then the resulting voltages across the armature windings are shifted by 120 degree.