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Simply speaking the answer is that you need to fulfill Hashem’s commandments – the right action at the right time and place. But still, we have the right to ask: why was there a difference in the commandment from one time to another?
The commandment to hit the rock in Beshalach involved the use of Moshe’s staff. Earlier in the sefer, we are introduced to the use of Moshe’s staff: "This staff you shall take in your hand, with which you shall do the otot (signs, miracles)" (Shemot 4:17). If we look at the use of the word ot/otot in the Torah, we may come up with a solution.
There are a few mitzvot that the Torah calls an ot: 1. Brit mila – "You shall cut the flesh of your foreskin, and it shall be as an ot between Me and you" (Bereishit 17:11). 2. At the burning bush – "Alas, the bush was burning in the flame, and the bush was not consumed … And He said: For I shall be with you and this is the ot that I will be with you" (Shemot 3:3,12). 3. Tefillin – "It shall be an ot on your hand and a remembrance between your eyes …" (Shemot 13:9). 4. Shabbat (which was first given in Egypt or at least in Mara (pre-Sinai)) – "But you must observe My Shabbat, for it is an ot between Me and you for all generations, to know that I am Hashem Who sanctifies you" (Shemot 31:13). 5. Marking the doorposts in Egypt with the Korban Pesach – "And the blood shall be an ot for the houses, and I will see the blood and pass over you" (Shemot 12:13).
What this whole list has in common is that they are connected to the period before the giving of the Torah. Chazal referred to this period as "before the speech" (see Chagiga 6a), i.e., the hearing of the word of G-d at the revelation at Sinai. Now we have a point to develop. The use of the staff for getting water from the rock was appropriate in Parashat Beshalach, which is before Sinai, and therefore it was the right system at the right time. After Sinai, Moshe is no longer to use the staff, as it is the time to use speech and only to use speech.

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