I was thinking of this question in chapter 91:

"Is that a usual tactic, by the way?" Harry said, voice still odd. "Carrying something large Transfigured into something small to use as a weapon? Or is that a usual exercise for Transfiguration practice?"

There are a scattering of other discussions of the usefulness of practice, and I thought there was a paragraph that explicitly stated the theory that Dumbledore had given him the rock when he knew that McGonagall was right there, so that Harry would come up with the idea of Transfiguring it, and would thus practice, but I'm not finding it easily.