Decanting is treated as a single ritual when it is really two unrelated tasks that happen to use the same vessel. Confusing them is why the practice has a reputation for being pointless.
Separating sediment is a mechanical job
Older reds throw a deposit of pigment and tannin that has bonded and fallen out of solution. It is harmless but coarse in the mouth and muddy in the glass.
The bottle is stood upright for a day so the deposit gathers at the base, then poured steadily in one movement until the sediment reaches the shoulder.
Any hesitation stirs the deposit back through the wine, which is why this pour is done slowly and never restarted. A light behind the neck shows the moment to stop.
Aeration is a chemical one
Young wines that taste tight or reduced benefit from contact with air, which disperses volatile sulphur compounds and lets aromatic compounds express themselves.
Here the goal is turbulence rather than care. Splashing the wine down the side of a wide vessel achieves more in a minute than an hour of quiet sitting.
The surface area exposed matters more than the elapsed time, which is why a wide-based decanter does something a narrow one cannot.
The two jobs pull in opposite directions
An old wine needing sediment removal is also the wine least able to withstand oxygen. Its aromatics can fade within twenty minutes of leaving the bottle.
Decanting such a bottle early, gently and close to service is therefore the standard approach. The container is a filter rather than an aerator in that case.
A young, structured red is the reverse and can sit exposed for an hour or more without loss. Treating both bottles the same way damages one of them.
The glass does more work than the decanter
Wine sitting in a glass is being aerated continuously by swirling, which exposes a thin film to air repeatedly.
For most wines this is sufficient, and the difference between the first and last sip in a glass is often larger than any decanting effect.
Where a bottle is being drunk over an evening rather than in twenty minutes, the wine effectively decants itself in the glass.
When decanting is the wrong choice
Sparkling wine loses its pressure and its texture the moment it is poured into an open vessel, and the bubble is most of the point.
Delicate older whites and lightly built reds can be flattened by air rather than opened by it. Their aromatic profile is the fragile part.
The reliable test is to taste the wine on opening and again twenty minutes later. If it improved, air is helping; if it thinned out, the bottle should have stayed closed.