I don't know why the MiG-21 specifically is like this, but I can think of reasons why it would be useful in other planes. Some aircraft (like the Citations I fly) have an aileron/rudder mixer that gives additive rudder for whatever aileron deflection you are putting in. I don't think it is linear, so it might make sense to make the curve progressively steeper (to a point) for larger control inputs. Again, I don't know why it is suggested for the MiG-21 though...
BeachAV8R