Supposedly it will help it heal faster, since you're spitting up some of the crap that your body is trying to expel in the first place, and because of that crap being in your body at all made you sick/your throat hurt. The more you can spit up the better, but don't go overboard with it :)
It depends. It will make you get better as you are expelling it. BUT, if it is coloured (yellow, green, brown) you will need to see a doctor to get a rx for an antibiotic. It means you have an infection.