Since Hashem gave us a brain, why did He need to give us a Torah?

L'Chayim
Yanki Tauber.

While reason is a very powerful tool, there is something in the human being that is even more powerful - desire, that creates an ulterior motive which causes one to reason in a particular direction.

When we want something badly, desperately enough, we grab our brain, twist its arm behind its back, and compel it to manufacture such gobilydoop that anyone looking at us from the distance of a generation or a culture away will exclaim, "What happened to that person? Did his brain leak out of his head?". Of course, our brain is right there inside our head. It's even working overtime. It's just that it's running in reverse. Instead of figuring out the truth, it's figuring out how to build a logical foundation for what we want should be the truth.

That is why Hashem gave us the Torah --- a set of truths.
To enable us to know when our brain is running backwards. If the end result of our reasoning and logicizing doesn't mesh with the divine truths in the Torah, then we are doing something wrong.