The angled position behind the gun comes with the "prone unsupported" position, where we use our non-firing hand to support the gun, with or without the sling. By moving our bodies off to the support side and drawing up the firing leg, we take pressure off the diaphragm, increasing comfort and allowing us to reach forward more easily with the support hand. It is physically impossible for most folks to get straight behind the gun when using the prone unsupported position.
Shooting "prone supported", with either a bipod, ruck or improvised support, is is no longer necessary to get off at an angle, since the support hand does not have to go forward with enough bend to rest on the ground while holding the rifle up.
As Buffalobob points out, it then has to do more with what one has learned and become comfortable with.
One major consideration to shooting prone supported is the the non-firing hand no longer fully controls the front of the rifle, and recoil will have a greater effect in displacing the gun during firing.
Because of this, when one is not straight behind the gun when shooting prone supported, we will often get horizontal displacement of our shot. It is quite common to see shooters that are using bipods for the first time to try to keep their traditional offset angle behind the gun, but this will usually lead to shots striking to the right of their normal zero point, and/or horizontal stringing. The straighter we can be behind the rifle that is not controlled by our support hand, the more consistent our results will be, both shot-to-shot, and when going from prone supported to other positions. Getting more directly behind the gun allows the body to absorb recoil in a straight line and thus helps keep shots on target better.
