There are three reasons why you would decant red wine.
The first is for show. Just because the customer wants to look good in front of his guests or maybe stupid.
You see young wines really don't need to be decanted and the guests might know this and thus the host looks uneducated about his wine. Let it go. Give the customer what he or she wants. The only comment I would make is that if the wine is a Burgundy red, a pinot noir of some quality then it is very delicate and I would not decant it. This will bruise the wine.
The second reason you would decant a wine if to give it air or oxygen. This will allow it to breath faster and open the wine. This should be done for heavy wines, which are high in alcohol. Most Italian wines and some American wines fall into this category.
The final reason to decant a wine is for sediment. Red wine over time will produce sediment and this is not desirable in the glass. You need to use a light source (usually a candle) to view the neck of the bottle to see when to stop pouring into the decanter to hold the sediment at the shoulder of the bottle. You will usually leave about an ounce to half an ounce of wine in the bottle with the sediment.
