You don't always have to decant a red wine: in most cases you can simply crack them open and enjoy. Some wines that feel closed, may need to open up and be exposed to oxygen. Some older wines may contain a sediment in the bottle which is unpleasant to drink. I took a trip to France and drank quite a bit when I was in Paris and many of the decanted wines had sediment and it was pleasant to see it at the bottom instead of getting a surprise at the end of the bottle. The sediment tends to sit in the bottom better when in a decanter as well.Decanting can also be a matter of personal preference. 

Older wines tend to fade much more quickly than younger wines so it's a good idea to decant them to separate them from their sediment. In these scenarios, it's wise to serve the wine within the hour. If you're the type of person that is never in a hurry when drinking their red it is perfectly ok to let the wine develop in the glass. The bigger the glass the better.It is important to keep in mind that all wines are different and they will call for different things. I have had wines that have sat for days and improved with time and then had others that won't last a day. 

A fun experiment can be to buy two bottles of the same wine, decant one and then drink the other right away. Be sure to have a small drink of water between to clean your palette as this can sometimes be quite hard.

We have a really nice Cabernet Sauvignon that opens up when decanted we highly suggest giving it a try.