Generally decanting is to leave out sediments in the bottom of the bottle. Decanting is also used to aerate the alcohol. The exposure to the large surface area of a decanter aids in this process and makes the alcohol more palatable, especially if it is young. I know this applies to wine but I am not sure about it applying to scotch since I rarely see deposits - but maybe it needs to be aerated. This is probably why you want a large mouth glass for it as well.
