You know, they aren't at all necessary. First, whites and roses do not benefit from a decanter. Second, there are different devices that will aerate the wine as it's poured thus not exposing the whole bottle to oxidation. This is what most wineries do for tasting. Then at the end of the day, the opened bottles left over are purged with argon and resealed.
I personally use an Aervana electric aerator/pump.
Now that I said that, aerators are good for younger wines that are full bodied (Cabs, Zins, Malbec, Petite Syrah and etc.) For more delicate reds like Borolo or Pinot Noir, decanting may be preferred. Also well aged reds like a Bordeaux or Rioja will benefit more from decanting as aerating can be violent and upset some of the delicate flavors gained from the aging process. Also, if you have an older wine with sediment, decanting is appropriate.
So, lastly, generally speaking, wine made in the US are made for immediate consumption. Wines made in Europe (especially France) are made to lay down in the cellar for a bit and allow the tannins to mellow out. So, following the information from above, if you are opening a nice Napa Cab from the wonderful 2015 vintage, use an aerator and enjoy. If this is a 2004 left bank Bordeaux (how about Pomerol?), then a decanter is your friend.
Now I don't want to put it out that I am providing disinformation, so I want to qualify this as "generally speaking " and not as absolutes. In fact, there are times when decanting an old and oakey Chardonnay is appropriate. Even after I said above that whites don't need it. There's always that "but sometimes". Knowing when and what is the key.