Purpose of Decanters?

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.

I can't imagine that there is any advantage to decanting distilled spirits. With scotch, you want to minimize exposure to air (oxygen); some people even inject nitrogen to displace the oxygen in order to reduce exposure once a bottle is opened. Decanters are strictly for appearances when it comes to distilled stuff. For wine, they are used to aerate the wine for an hour or so prior to serving.

To pour older wines off of any sediment in the bottle and to give younger wines a vigorous aeration to help them open up.