Answer:
The reason for writing it.
Explanation:
The reason for writing said essay is also the purpose of the essay which could be considered a (although small) summary of it. The tone of it wouldn't matter because the tone you use isn't a reason for the purpose, as are the details and materials, which would be considered the purpose of the essay. And the people reading it are just the audience, so they really have no say in the essay regardless.
