Why did Moshe need to cry out to Hashem? Why did Tefilah alone not suffice?
Seforno: Because some of the frogs (crocodiles) would remain in the Nile, and Hashem does not generally work in halves. 1
Sifsei Chachamim (to 8:6): Because Tefilah needs to be audible. 2 Therefore, due to the terrible racket caused by the croaking of millions of frogs, Moshe needed to scream in order to hear himself.
Moshav Zekenim #1: Because he wanted the magicians to hear him above the frogs' croaking (otherwise, they might think that he removed them through magic, and not through Tefilah).
What are the implications of "Asher Sam l'Pharaoh"?
Oznayim la'Torah: It implies that Moshe only Davened for the removal of those frogs that afflicted Pharaoh, but not of those that remained in the River Nile.
QUESTIONS ON RASHI
Rashi writes: "'And he cried out' - ... that very day, that the frogs should be cut down tomorrow." What forces Rashi to explain this way?
Gur Aryeh: Rashi (to 8:5) explained the word "l'Masai" to mean that Moshe would pray immediately, that the frogs should die at a later, designated time. Certainly, Moshe needed to follow through, to show Pharaoh how Hashem kept His word.
