Why did pharaoh not want the frogs to be immediately removed?Why did he request that they be dealt with "tomorrow"
And Moses said to Pharaoh, "Boast [of your superiority] over me. For when shall I entreat for you, for your servants, and for your people, to destroy the frogs from you and from your houses, [that] they should remain only in the Nile? "
And he [Pharaoh] said, "For tomorrow." And he [Moses] said, "As you say, in order that you should know that there is none like the Lord, our God.
Rav Hirsch points out that Par'o was actually trying to outsmart Moshe. He knew that the plague of blood had ended by itself. He thought that if he said today, the plague would have ended anyway. By saying tomorrow he would be able to claim that Moshe was just a powerful magician and the frogs would disappear immediately, because Moshe would have set up the removal already as soon as he was called to Par'o.
The motive for Moshe making the exit of the plague itself into an אות, and not simply undertaking Pharoah's request, lay in the request itself. Had Pharaoh been already convinced, he would simply have let the people go. But as it was, it was apparent that all he wanted was to be freed from the plague. That is why the departure of the frogs was itself to be a further instructive אות; perhaps this would produce the desired result. Accordingly, the frogs are not simply to go back from where they came, that could be taken as merely the cessation of Hashem's activity and a return to normal conditions. They are to die, but as emphasized repeatedly ביאר תשארנה, not all of them, that could again be taken as some phenomenon of Nature (cf. V9).