Not every wine needs to be decanted, but many can really benefit from it. There are two good reasons to decant: 1) to let your wine breathe in hopes of softening tannins and bringing flavors and aromas in focus, and 2) to remove sediment from older wine.
For big, younger wines, a quick, aggressive “flash decant” will do exactly what an aerating device does by introducing important oxygen and smoothing it out. Letting it breathe beyond that will help even further!
Older wines may have loose sediment that needs to be decanted away, requiring a slower and more attentive decant.