1) Older red wines often have crystalline deposits in the bottle. These occur naturally, and don’t affect the taste of the wine, but it can be a bit disconcerting to have these in the glass. Traditionally, when decanting, the neck of the Bordeaux-style bottle is meant to capture these deposits. It’s OK though to use a stainless steel pour spout and filter to perform this function.
2) Wine interacts with oxygen, usually for the better when you open the bottle. Pouring the wine into a decanter accelerates the oxygenation of the wine, helping your older red wines (and younger ones too) to hit peak taste as quickly as possible.
3) There are some very beautiful crystal decanters; it’s nice to be able to put yours on the dinner table and finally use it.