Skill synergies.

I only see this with goblins because they are cowardly, and their attack stats aren't very high, so it works for them, but there are too many combinations for melee units, while there isn't much room to maneuver for ranged combat.

I once thought about combining Pack Tactics with Orcish Bloodlust – Pack Tactics allows all units to attack in a group, and Bloodlust makes it not seem overpowered.