Why do we only use so little of our brain?

The human brain is goal oriented and uses what is necessary at the time. Even a supercomputer uses little of its capacity to calculate 2 + 2 = 4 (the operator may be joking?). While determining whether it is safe to cross the street would be a bad time to recall everything we have learned. Even though we may never remember verbatim that we have read a book, usually within a page or two we can 'recognize' that we read it long ago. The brain stores a great deal of info that we rarely need to access. Having a huge over-capacity ensures that we never run out of space to store more, especially if we never cease to study and learn more. It is also easier to 'organize' info in a spacious room than within cramped quarters.

We don't. It's just some old misconception - pretty much like the "bumblebee can't fly" garbage.

We don't use of all our brain at the same time though, but that's another story.

The "reasons" you mention are not given as much as bred. The appendix for example, or hairloss or short eyesight. I understand what you're saying, I just can't agree.

We use 100% of our brain - we just don't use it all at once.

At any one time, about 10% of the neurons in our brain will be firing (any more and you're having an epileptic fit).

But averaged over the whole day, we'll use the whole brain.

I think its an evolution thing, the scientists reckon that we have started using more of our brain in the last millinium so probably in 5000 years we will be on full capacity.

It's a myth, they busted it on QI. We actually use most of our brain, just different parts for different things.

We don't know yet. We COULD be using it but aren't smart enough to know what for.

this theory is actually contested. please google your subject