Why did Hashem see fit to appear 'in Person' by this miracle?
Ramban: Because the miracle of the roving well remained with them for forty years. 1
As stated in the Midrash Rabah. Hashem did the same thing by the Manna (See above, 16:7), which fell for forty years.
What is the fact that the Torah writes "Vehikisa ba'Tzur" (and not al ha'Tzur) coming to teach us?
Rashi: It teaches us that Moshe's staff comprised sapphire, something that was able to split rock.