Originally Posted by Robert B. Mercer

I believe that the CAN adds meaning to the sentence.

It changes the sentence from the speaker PROMISING, to

just SAYING that he is ABLE to promise... but at that moment he IS NOT PROMISING.

I CAN PROMISE you that you can buy the car for $10.

In this example, if you listen closely, he IS NOT promsing you, at that moment, the car for $10... he is only SAYING he COULD, if he chooses to.
I study Spanish means I am doing it right now, I can study Spanish announces that I am capable of it.. but I might never do it.

I can promise you I will never hit you DOES NOT promise I won't hit you... it is just a very, very, very sneaky way of hoping you hear it that way.