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
Other commentaries explain that it was due to the terrible racket caused by the croaking of millions of frogs, which made even hearing oneself difficult. 2