Prana,
I think the purpose for letting go and letting God, as some people say, is to simply take responsibility and thoughts off one's shoulder.
Myself, I can feel when I am shouldering too much and will try to let go. My personal bias is to let go and let the universe do whatever it wants with certain issues. And who knows? Maybe my unconscious mind and maybe even other forces that exist in the universe (???) may help.
The way I view it, there may be no free will or perhaps not even the need for it. Sometimes I just shut off, let go, and let the universe engage in the act of being me rather than feeling I have to do and be and have some responsibility all the time.
Sometimes its relieving just to let go. As Alan Watts said, you are the experience the universe is having. You're an aperture through which the universe can experience itself. I may not be true, but it sure induces one hell of a trance. 
