Why did Hashem find it necessary to appear in Person by this miracle?
Ramban: So that the miracle of the roving well hould remain with them for forty years. 1
As stated in the Midrash Rabah. In fact, Hashem did the same thing by the Manna (See above, 16:7), which also contimued to fall for forty years.
Why does the Torh insert the (otherwise superfluous) words "Hin'ni Omeid Lefanecha "Sham al ha'Tzur"?
Targum Yonasan: Hashem was telling Moshe that he should go to the spot where he woujld see the mark of a foot on Har Chorev, 1 which is where he would strike the rock.
See Peirush Yonasan.
Why the Torah write "Vehikisa ba'Tzur" (and not 'al ha'Tzur'?
Rashi: To teach us that Moshe's staff comprised sapphire, a material that was able to split rock.
