by MountainRoot
I guess you mean to CHOOSE to engage him although you have lower threat or he is about to go to the other players is the enemy engages automatic.1 - Because if you let the enemy he will go to your companion and he can't deal with him.
2 - Because you are in a position to beat him now and don't know what the future cards will bring.
3 - Because even if you can't beat him you still have to remove it from the stage area to lower the threat and quest successfully.
4 - Because you have to deal with a really bad effect he has or in order to deal another one.
Example: Core Set, Journey down the Anduin, engage an enemy in order for no other enemy other the goblin sniper to be in the stage area and in later rounds engage him also...