Why is תפילה important?

(2) Shimon the Righteous was one of the last of the men of the great assembly. He used to say: the world stands upon three things: the Torah, the Temple service, and the practice of acts of piety.

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(1) ועבדתם את ה' אלוקיכם, the manner in which you will demonstrate that you indeed serve Him is by destroying all the idols as mentioned in the previous verse. As a corollary of having destroyed idolatry and its representatives you will not need to worry about anyone in your midst trying to convert Jews to such alien religions and to thereby estrange them from serving Me.

​​​​​​​ “You shall serve God”—“service” is prayer (Bava Kama 92b)—“and He shall bless your bread and your water, and I shall remove illness….” In other words, you will be healed through bread and water. As a result of their being blessed from the root of everything—i.e., the Word of God/prayer—bread and water will have the same healing power as the plants.

(2) It is important to appreciate that the power of prayer is such that it can bring about changes in the laws of nature, save people from mortal danger, and bring about cancellation of evil decrees by G’d. We know that it can effect changes in the laws of nature from Yitzchak whose prayer in the presence of Rivkah brought about the new-found ability of Rivkah to conceive (Genesis 25,21). She had previously been barren, and our sages explain that this was so as G’d wanted to encourage Yitzchak to pray to Him to reverse this situation. (compare Shir Hashirim Rabbah 2,31). Our sages explained to us that the barrenness of our matriarchs was due only to the absence of the prayers of either themselves or their husbands.
Proof that prayer can save one from mortal danger is found in Psalms 107,23: “the ones who descend to the sea in ships.” We know that from when Jonah prayed inside the fish, surely a mortal danger if ever there was such. Not only this, but David continues in that psalm describing how the mariners in time of a storm pray to G’d Who saves them from these dangers. As a result of their prayers G’d silences these turbulent waters (verse 29).
Proof that prayer is able on occasion to reverse evil decrees by G’d is found in Isaiah 38,8 where G’d tells King Chizkiyah that his prayer has been granted that he will enjoy a 15 year extension of his lifespan after the prophet had told him in the name of G’d to make his last will and testament as he would not recover from his sickness. (Compare Berachot 10 on what the king told the prophet.) The Talmud tells us not to despair even if the sword is poised to fall on one’s neck, that G’d is still able to help. They quote Job 13,15 to prove their point. Incidentally, our sages there reveal that the power of prayer exceeds the power of prophecy.