Seeing a wine steward decanting a bottle of wine in a restaurant can look 'showy' or a bit pretentious. And a wine decanter can be a beautiful thing to display, with or without wine in it. So why decant a wine?
The simple answer is that decanting a wine allows it to 'breath' much like the previous discussion on swirling a wine glass. By pouring a bottle of wine into a decanter (or any glass vessel), the wine gets an opportunity to quickly release any volatile compounds that have built up in the bottle and get exposed to air. And this really can make a difference in wine.
If you haven't tried decanting its well worth it. Immediately after opening a bottle of wine (particularly a red wine), pour yourself a small taste in a glass so that you experience the wine right out of the bottle. And most good wines will be fine right out of the bottle. But then give the wine in the decanter thirty minutes to an hour before going back and trying it again. Most often, you will find that the wine has smoothed out and is a bit softer, not quite so sharp as that initial taste from the bottle. And, you are more likely to detect some of the subtile flavors in the wine.
Decanting is particularly effective with young red wines. Exposure to air in a decanter for one to two hours can make a significant improvement. But, with an older bottle, decanting my not be necessary at all, and it can even degrade some of the delicate nuances of a fine wine.
Decanting was especially effective after recently opening a rather young bottle of red wine. After the first taste right out of the bottle, I immediately said 'no' this isn't a very good bottle of wine. So, I poured it in a decanter and left it for a couple of hours. Upon returning to the almost forgotten decanter, I poured another small glass and noticed a significant improvement. Gone was the sharp bitter and highly tannic (dry) taste. The wine had calmed down, smoothed out and was then quite enjoyable with a meal.
Another item to consider along with a wine decanter is a small wire-mesh filter designed specifically to be used for wine. These can be very effective in removing any sediment that might have remained with the wine in the bottle. Just pour the bottle of wine through the filter as you decant it.
One of my favorite lines about wine also applies to decanting. It's said that a wine cellar is not a wine hospital; it won't make a bad wine better. So too, decanting will not magically turn a bad bottle of wine into a good one. But, you may be surprized how decanting can make a nice wine even better.
So, if you don't have a fancy wine decanter, don't worry. Any wide glass vessel that will give the wine exposure to air will work. Just don't choose a plastic or metal vessel that can impart other flavors into the wine. And, don't decant too long. A wine can go flat and loose much of its flavors if left exposed to air for too long. Just try a sip right out of the bottle to establish a starting point, then decant and periodically re-visit it. You should notice an improvement. Cheers!