Good question, and one I've wondered about too. I have no written references, so what I say is open to correction. However...
My understanding is that Va is a little too slow for best performance in turbulence. Yes, it protects the aircraft from structural damage due to excessive g load. But you could inadvertently stall in severe turbulence, which is just as bad.
Rough air penetration speed, or turbulence penetration speed, is around 5% to 10% faster than Va to reduce the risk of stalling. I don't know the precise calculation. The manufacturer expects the pilot to maintain an
attitude in turbulence, specifically not to try to maintain an altitude, and this will reduce the chance of structural overload.
Here's what the good people at FAA.gov require (gusts are assumed to be 50fps according to rule 23.333, and the speed Vc is equal to 33 times the MTOW (lbs) divided by wing area (sq feet) according to rule 23.335.)
(d) Design speed for maximum gust intensity, VB. For VB, the following apply:(1)
VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:
(i) ng is the positive airplane gust load factor due to gust, at speed VC, and at the particular weight under consideration; and
(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.
Couple of extra things, from others' posts above:
- Va protects against a single full and abrupt control input. It does not protect against multiple controls used simultaneously, or against one control input made multiple times.
- If using aileron and elevator together, the rule of thumb is that the actual 'g' limit is only two thirds of the basic limit. This is called the rolling g limit. Note also that for aerobatic aircraft, the entry speed for a flick roll is very much lower than Va.
- Va does not protect against multiple sequential inputs of the same control surface. If for example, at Va, you apply full left rudder then immediately full right rudder, you can expect to break something.
- Va does not necessarily protect against full forward stick. If you want to mess about with high negative 'g', Va is not the number to use.
- Va is not necessarily about the wing. The wing spar is usually over-engineered. Va might be limited by the tailplane or fin strength, or even by the strength of the aft fuselage (which takes a huge bending load at high g.) It is possible that the first item to fail might even be the engine mounts, or other high-load points inside the fuselage.
(Edit - I don't know much about the C210. My comments in the last sentence are for light aircraft generally, not the 210.)
Cheers,
O8