There are actually more reasons to flex a ski.
(i) to find out how stiff the ski is
(ii) to see the flex curve of the ski
To evaluate the latter the ski should have the boot in the binding. Even if it´s not a precise simulation (skiing with speed and the forces generated influence the way the ski and plate flex and the binding works) it´s a good help.
The best way to see the curve is to place the angulated ski (like being under a good edge angle) on a plane surface and to press it till the whole length of the edge is in contact with the surface.
A trained eye can see quite a lot. A friend of mine who had developed race skis of his own a couple years ago (and the trained eyes on this occasion) used such a wood board with interesting results.
We even tried to press the ski into sand to see the curve.
I don´t think you see a lot of naked guys trying to press a ski into the sand on a nudist beach like we once did.