Wines showing important deposit are better served after being decanted. The deposit is formed during the ageing process of many red wines, especially those with high tannins : Bordeaux best Cabernet Sauvignons for instance.
Some young wines also benefit from the aeration that occurs by being decanted, though this can be done easily by swirling the wine in a glass, or waiting a few minutes after serving.
Note that airing your wine before service by just opening your bottle will not enhance it. Indeed, too little of the wine is in contact with the air and it will have amost no effect. This is why decanters usually offer a large surface of wine in contact with the air.
How to decant a wine ?