Decanting a wine, or pouring it delicately into a carafe or decanter, is a desirable operation especially in the case of aged red wines, which tend to strip of color over the years and form the deposit.
The use of the carafe serves two things: to decant the wine and oxygenate the wine. In some cases, in fact, even if there is no foundation, it is equally necessary to pour the wine into a carafe because, having remained in a "reduced" environment inside the bottle, it needs to release its own perfumes. The bottle should not be shaken so as not to mix the deposit with the rest of the wine. While the wine is slowly poured into the carafe, sudden movements must be avoided in order not to put the suspended wine deposits. For this purpose, it is important to place a light source, such as a candle, under the neck of the bottle, so as to see the approaching deposits and stop pouring in time. The wine must be reached very slowly, creating a spiral along the neck of the carafe that will facilitate wine oxygenation, after it has been forced for a long time in an air-poor environment such as the bottle. On the other hand, excessive oxygenation could completely fade the aromas of wine and of a very old wine, whose structure is now very fragile. In conclusion, and once more, a fixed rule does not exist. Only experience and some specific advice make you decide each time for the best.
Source: Ediz.Giunti, Vino-Il Manuale del Sommelier