>>/37753/
It's not necessary, but it makes a big difference. For young big, bold, tannic wines, you can decant and get it to come together pretty quickly.

For old wines with a lot of sediment,  decanting helps separate it out so each glass isn't full of grit.

I drink a lot of Pinot Noir so it's not strictly necessary (I usually just open bottle and let it slowly open up over a few hours), but sometimes if I'm going to a tasting where I want it to be at it's best quickly, I will double decant which is pour bottle into decanter and then back into bottle.