If an enemy is positioned behind another enemy than your ranged fighter will get a penalty to hit, if he tries to shot the guy in the backrow. Bullseye reduces said penalty and therefore makes it easier for your ranged fighters to snipe the opposing backrow. Should you plan to use your ranged fighters to counter other ranged fighters this perk can be very useful. I think it is a solid pick for every dedicated crossbow or archer build. Hope this helps.