You may here me have read that I sometimes recommend decanting a wine. Some wines really need it. The purpose is to aerate the wine. If you have ever seen someone swirling wine in the glass or 'letting it breathe' or waiting for it to 'open up.' They are all doing and describing the same thing. They are trying to get oxygen into the wine to help bring out some of the layers of flavors. It also allows a little of the alcohol to evaporate and bring out the aromas so that you can smell what you are tasting. The oxygen will also help soften the tannins in the wine. This is that dry puckery sensation you get when drinking some red wines. So when you first taste your wine from the bottle, if it is not what you expected, try aerating it and see if you get something much different. Don't decant a white or rose wine.
There is also another good reason to decant a wine. If you ever find yourself in the position to impress guests and don't want to advertise a bottle of $1.99 wine or a box wine, go ahead and serve it from a decanter. There's a good chance they won't know.