va'eira

The Posuk says ויאמר משה לפני ה' הן אני ערל שפתים ואיך ישמע אלי פרעה. Moshe asked 'ה, I have a speech impediment how will Pharaoh listen to me? Hashem tells him, don't worry you're going to be a lord over Pharaoh & Aaron will be your interpreter.
Why did Moshe have to have a speech impediment and need to have an interpreter? As 'ה told moshe when He first appeared to Him, "who is the one that gave a mouth to a person or made one mute made one see or blind is it not I, 'ה?"
Why didn't Hashem just cure him?
The Posuk says Hashem said to Moshe, tell Aaron to take his staff & stretch his hand over the waters in Mitzrayim to change the waters to blood.
Rashi says Aaron had to hit the water & not Moshe  because the water protected Moshe when his mother put him in the river.
The שיטה מקובצת brings down a Gemara that says, "the well that you drank from, don't throw stones in it". Something that benefited you, don't disparage. Don't pay back kindness with evil.
The ריף says that if that's how we have to treat inanimate objects that have no feelings, how much more so do we have to be cognizant of the feelings of our friends & the people around us whom we benefit from.
Maybe this is how we could understand why 'ה didn't just cure Moshe & let him go to Pharaoh alone. He needed to have Aaron there to do the talking. Moshe grew up in Pharaoh's palace; he lived there, and Pharaoh fed him & took care of him.
The same way he couldn't hit the water that protected him to start the דם, he couldn't be the one to talk to pharaoh, who had cared for him to start the Makos. Even though they're about to destroy his whole country, a Rasha still deserves some Hakaras Hatov. 
It goes without saying how much more so we have to recognize the benevolence of 'ה who gives us all good & who always takes care of us, that we shouldn't do anything that is abhorred by Him, and do only good, to somewhat repay his abundant kindness!