The Purpose of Prayer

Across the street from a yeshiva stands a billboard that doesn't belong there, and one man decides to pay a lot of money to make it go away. What he puts up in its place puzzles every driver. It fits Bnei Yisrael at Refidim. Why would Hashem lead a people dying of thirst into such a test? And why does Moshe, who always davens, not daven for them? The Or HaChaim finds a cornerstone idea about what tefillah is for, beyond getting what we ask.