The process of decanting a red wine allows it to breathe and, in turn, livens up the wine. Removing the cork from the bottle will not have the same (if any) affect.
It separates mature wine from the sediment; it mellows and 'brings out' younger reds. Contact with the air, in a sense, 'accelerates' the ageing process.
Decanting should take place a couple of hours before drinking but less, perhaps, for older wines that can fall away through the shock of air contact.
Before decanting, ensure that the glass decanter is clean and soap-free.
Pour the bottle at a reasonably rapid rate, being careful towards the end to ensure that any sediment remains in the bottle.