The Medrash (Devarim Rabbah 2:10) states: “There is a tefillah that is answered by Hashem before it is uttered with the mouth, as is stated (Yeshaya 65:24): ‘And it will be before you call out and I will answer.’” The Shem M’Shmuel (5676) asks what benefit there is to this. Hashem wants to hear our prayers and He desires to hear our voices, as is stated in the Zohar Hakadosh (Chelek 3, 200B). The Medrash (Bereishis Rabbah 45) further states that the reason the Imahos were barren was because Hashem wanted them to daven to him. If so, how is answering prayers before they are uttered a sign of love and what benefit does this have?
He answers that only Hashem knows a person’s thoughts. The Zohar Hakadosh states that even ministering angels cannot know a man’s thoughts, and prosecuting angels certainly do not. Therefore, when a person decides that he wants to daven to Hashem, no entity knows about it besides for Him and no destructive force can obstruct him. Once the prayer is said aloud, however, the prosecuting angels do know about it and they are able to bring up his past transgressions in an attempt to stop his tefillah from being accepted.
Since Hashem always rules fairly, He allows the prosecuting angels to bring their claims. However, since He loves Klal Yisroel, He wants all their tefillos to be heard. Therefore, He accepts the prayer even before it is uttered, before the damaging angels have a chance to obstruct it. And once the prayer has been accepted, it cannot be challenged.
