Why did Hashem remove the wild animals and not kill them (like he killed the frogs)?
Rashi and Ramban: Because then they would have derived benefit from the animals' skins. 1
Oznayim la'Torah: Because, as opposed to the frogs which were created specifically for the plague of Tzefarde'a, and which were therefore superfluous, Hashem had gathered them to Egypt from all over the world, and now that they had performed their task, He sent them back to where they came from.
Ramban (citing the Midrash): And it was Hashem's intention that the plagues should cause the Egyptians to suffer, and not that they should benefit from them in any way.