Thanks to Caverna for answering this.
The intent is to give the players some additional options to attack more aggressively or defensively. For example, I had a situation where one of my players Mage was involved in combat and was being overwhelmed by multiple opponents and she chose to fight defensively (narratively, "I'm going to back away and focus on defending against any attacks") until her companions could assist her.
What I have allowed is the players to call this at the start of each turn which means I can then apply the advantage/disadvantage effect for that individual turn rather than having to remember to apply it the next turn if one person has acted after another. Even though initiative is only rolled once which sets the turn order for the entire combat it still made it a little easier to remember during play.
The key thing here is the player should gain the benefit of the advantage or disadvantage and so should their opponent (even if this falls into the next turn due to turn order).