The motivating factors of AI are:
(1) As you mentioned, to learn about the human brain. If we can make a system that behaves like the brain in a certain way, then we know that the program running in the brain is likely similar to the one we created.
(2) To help us do stuff. Basically creating a really smart system that can think on its own, learn from experience and reason. Machines do things for us, and if they had these abilities they'd be able to do them better.

One thing to note is that AI hasn't really come close to actually mimicking anything similar to a human brain. Specialized systems have been created, but no universal system that can really think like a human can. Sentient robots that might take over the world are still VERY far off.