Boons have the most value when your chance of success is at 50%, meaning attack and defense are evenly matched. Their effectiveness diminishes as your chance of success moves towards the 0% or 100%. Therefore boons are best used to either even the odds in a close fight, or to give yourself an advantage in an even fight.
The graph to left shows how the effect of a boon scales for various attack and defense rating. Dark blue is where the boon is least effective. Dark red is where the boon will be most effective. If you look closely at the dark red band, you can see that boons are most effective when your chance of success is between 40% and 60%. The more quickly the color shifts, the quicker the boon becomes less effective.