Va ist the stationary, clean stallspeed for the positive maximum g-load at MTOW. If your plane is slower than Va, it will stall before your g-loads reach the limit value, if your plane is faster than Va, (or lighter than MTOW) you can exceed the g-limits by applying full up elevator.
Va is calculated directly from the 1g clean stall speed at MTOW by multiplying with the square root of the limit g´s.
Because Va is the natural limit speed for applying full up elevator, it is defined also the limit speed for any other full control surface deflection. (so you design all controls for the given Va value)
Vra is a performance value selected by the manufacturer, without any direct physical relation to loads or aerodynamics. It must be choosen smaller or equal to Vc. Vc can also be coosen by the manufacturer, but it must be greater or equal to an emperical calculated speed, depending just on the wing loading (JAR/FAR 23, small aeroplanes and commuters) or must provide a safety margin to
Vb (JAR/FAR 25, large aeroplanes). Once you have choosen Vc, you must use it to calculate gust loads.
Gust loads are not static aerodynamics, so the Cl max value for the airfoil (and hence the wing) is greater than the static one. Therefor at Va you can experience gust loads, that are greater than the limit g-loads, depending on various parameters.
Quite similar to Va,
Vb ist the speed at which the maximum positive gust leads to limit g-loads.
For most transport airplane the (normally assumed) positive 50 ft/s (JAR/FAR 25 : 56 ft/s) gust does NOT exced the g-limits at Va, so
Vb is greater thav Va, but that does not naturally apply to all aircraft. You calculate your g-loads at Vc for the 50/56 ft/s gust, which are greater than the
Vb limit loads (because Vc is greater than
Vb but the gust is the same), and design your structure to carry this load.
Afterwards you choose an additional safety margin and define Vra as the speed, up to which the plane can be flown in strong gusts.
So to answer your original question :
If you want to sell a plane, it must be fast. Therefor you want to define Vra as large as possible.
On the other hand, you don´t want to have to high limit g-loads, because this makes your plane heavy, so mostly only the mimimum required value is choosen, leading to a relatively small Va.
On almost any large plane
Vb ist higher than Va, and you choose Vc even larger. Same applies for the minimum required Vc for small aeroplanes. And you dont want to choose Vra with too much nargin to Vc, because you can´t sell a slow plane.
So Vc is larger than Va for any conventional plane, and safety margins don´t sell, therefor Vra is also normally higher than Va.