Oak barrels, new or old, have the key role of microoxygenation. In simple words, wine is allowed to come in contact with as much oxygen as needed to avoid the phenomenon of reduction but also to stabilize the color and soften the tannins (in red). When the barrel is new except for oxygen, it gives the wine the characteristic aromas of vanilla, tobacco, caramel, nuts, etc. This ability fads gradually after the first use, until it is completely lost (approximately after 3-4 years of use) . If a producer wants this aromatic character in his / her wine, he / she should, after 3 years, get a new one. But if he simply wants the barrel as a pot and exploits its porous nature, then he can very well buy a used, old barrel at a very low price. This is very common in Bordeaux. For example, Chateau Desmirail at Margaux, gives its old barrels to other local producers or even distillers.
Wood tends to absorb a small amount of the liquid that comes in contact. This has the effect of giving some aromatic characteristics to the next wine or distillate that will mature in the barrel. It is very common for some Whiskeys to mature in Sherry or Madeira barrels. This happens in aged Greek tsipouro as well that can be matured in a barrel from Vinsanto.
Finally, even when the barrel has absolutely no use in wine or spirits, it can be useful as a piece of furniture. With some transformation and a little imagination it can become a table or a chair and thus "live" almost forever.