By way of background, recall, that God frequently introduces prophetic communications thru symbolic examples. Thus Jr01-13 God showed a burning pot in the north to symbolize the prophecy that destruction (burning) would start in the north. Similarly, God told Hosaah to name his children no-mercy and not-mine to symbolize that God would not any more have mercy on the Jewish people and not consider them my nation.

In Nu20 God symbolically wanted Moses to speak (not strike) to the rock who would then give its waters. This would be symbolic of Moses speaking, not striking, the stone-cold Jewish people who would then comply (like the rock).

The symbolic method used here is called anthromorphism - the substitute of inanimate, plant or animal objects to symbolize humans. It is a very common method in all cultures and a powerful symbolic method. This command by God that Moses should engage in symbolic exhortation, if accomplished, would achieve sanctification of God's name since the people would comply not rebel against God.

Advanced Rashi:Any symbolic assertion such as the above can always be supplemented with further support and proofs. The use of rock to symbolize cold people is explicitly found in verse Ez11-19 which states And I will give them one heart, and I will put a new spirit inside you; and I will take the stone heart from their flesh, and will give them a heart of flesh;