Why pour a perfectly good bottle of wine in one of those contraptions that wine stores like selling us, Not only do I need "special" glasses but I also need a decanter.
There are two reasons for using a decanter. The first one and most common is that the wine is still young, usually “big”, tannic, high in alcohol, these wines will taste much more subtle, more elegant after an hour in a decanter where they had time to breathe, open up. I strongly recommend decanting big wines like Zinfandel, Shiraz, Cabs, but also lighter wine like Pinots. Most young wines get better after a little trip to the decanter. Remember to taste the wine before it goes in and after it comes out to compare and see if you like what aerating did to the wine.
The second reason for a decanter is to decant an old wine. This is usually done very slowly and the reason is to remove the sediment that have accumulated in the bottle over the years. Remember to do this very slowly and to drink the wine immediately, most older wine (20-30 year) will not benefit from aerating, they will usually go “flat” in 30 min, too much air will damage the wine.
A good resource to look at is the YouTube video from Tarbell here.