An important reason to use decanters is to separate the wine from the impurities in the bottle. This impurity is not a dirty thing, but a crystal of Tannin and pigment, which also forms during the wine making process, and some gradually after bottling. Especially high-grade red wine, often seen, this is pure ingredients, high concentration of proof. Although this can also be drunk, but it will be a bit bitter, will affect the taste of wine, so, still use decanter to separate them. We see that when professional sommeliers change bottles, they often do so on a light source, such as a candle, to get a better look at the impurities.
