Decanters were developed long ago when wines were unfiltered and therefore had excess sediment and lees in the bottom of the bottle. In order to limit the sediment in the consumer's glass, the decanter was used as the primary recipient for the wine. Decanters are still used for aged wines, and wines with traditionally more sediment like Barolos, the Grands Crus du Médoc, old Ports, etc.
The most prestigious wines are often unfiltered because this process can remove some of the best aroma and flavor compounds. For these wines a decanter may be used. For very old cellared wines, pour the wine gently by letting it slide slowly down the side of the decanter. For bigger, younger more robust wines, decanting can be an opportunity to further aerate the wine by pouring it more vigorously.