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 (on Pasuk 6): 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: He wanted the magicians to hear him above the frogs' croaking [lest they think that he removed them through magic, and not through Tefilah].
What are the implications of "Asher Sam le'Par'oh"?
Oznayim la'Torah: It implies that Moshe only Davened for the removal of those frogs that afflicted Par'oh, but not of those that remained in the River.