When do you need to decant wine? Here are 3 reasons. And, do not fear, it does not have to be a formal occasion requiring specific attire or any equipment beyond a corkscrew and a glass vessel.
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Old Wines
Old wines often have a significant amount of perfectly natural, harmless sediment at the bottom. It's just not very pleasant to drink. You want to stand up your bottle early in the day (or the night before) to let all the sediment collect in the bottom. Then you slowly pour the wine into the decanter until all you have left is the sediment at the bottom and a small amount of wine that you have to sacrifice to the Gods of the Wine Cellar.
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Young Wines
Young wines that maybe could use more time in the cellar can benefit from decanting. Often a wine that is extremely powerful right after you pull the cork can soften up after being out in the air for a while. Give it at least an hour, and taste the difference.
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Wines With Awful Labels
Maybe you're fond of a wine that has an unfortunate name or label. I found a collection of examples on the internet so disconcerting that I had to censor the image. Though I encourage you to stand behind your wine choices, having a decanter around can solve that problem. And it looks nice on the dinner table as well.
Happy decanting!