AP accumulates during combat however I have yet to see a specific use for them. I see no weapons, signs, or skills that state the need for them. I do see throughout application of the aforementioned that AP will increase during combat as well as drain out of combat.
From the article Adrenaline - how to gain it and what is it used for:
Adrenaline on its own doesn't change anything. But it can cooperate with many skills, modifying them or increasing their effectiveness. Most important relations with adrenaline have:
Undying skill from the Combat category - Thanks to this skill when you lose all health points you don't die immediately, adrenaline points can be used to keep Geralt alive.
Rend skill from the Combat category - Adrenaline points can increase the amount of damage dealt even by 100%.
Deadly Precision skill from the Combat category - Adrenaline points increase the percentage chance of instantly killing the opponent.
Flood of Anger skill from the Combat category - Having enough adrenaline points allows you to activate the witcher sign on the highest level possible and increase the attack strength.
Hunter Instincts skill from the Alchemy category - Having the maximum amount of adrenaline points will increase the damage of critical hits. However, it will work only if an oil effective against a specific monster type has been used.
Rage management skill from the Abilities category - It allows you to activate witcher signs even if you don't have enough stamina. Adrenaline points will be consumed instead of the lacking stamina.
Focus skill from the Abilities category - One of more interesting skills. Thanks to adrenaline points Geralt can deal more damage and the strength of his signs increase.
Various other sources also mention a general damage increase the more adrenaline you have. From a post on CD Projekt Red Forum :
Each point of Adrenaline increases your sword damage by 5% So with 3 Adrenaline points your max sword damage is increased by 15%.
Finally, some skills such as Whirl can be maintained with the use of Adrenaline once Stamina is depleted.