What is the purpose exactly of a ‘Control Check’?
Technically speaking, a control check is part of any pre-flight. 

In the case of Qantas 32, one engine had exploded, punching holes in the wing and setting off a host of warnings, so there was a reasonable expectation that some of the controls might also have been damaged. 

What the pilot did was put the plane through some basic maneuvers to see how the controls responded in their now potentially damaged form. And found that roll rate had been affected, something he factored in on the subsequent landing. He also found that pitch control had not been affected, which meant he could make a reasonably conventional approach to the runway.  

Better to find out how much the controls had been affected when there is time and altitude to recover from an unexpected response, than to find out on final approach, where the margin for recovery is very slim to none at all. 

During flight, this is not normal. It is prudent if an inflight incident may have damaged some of the controls, so that the pilot knows the level of controllability they have to work with.