The answer is probably easy but we’ll take a shot at it anyways.
For boss fights or rare enemies you will probably want a 1H and a Shield. I’ve also used a one handed weapon and a shield when I run out of potions and I’m lazy or when I am fighting large groups of enemies that are around the same difficulty as me.
Obviously, you won’t need the survivability if your gear and out level are far above the enemies you are fighting, however, if you have crappy gear and are around the same level or lower than the enemies you’re fighting you’ll most likely appreciate a sword and a shield.
On the other hand, whether you Dual Wield or use a 2 Handed weapon doesn’t matter, the result is you have less chance of blocking. The reason you would use these setups is if you want to cause a massive amount of damage. Usually this is used for clearing enemies where survivability isn’t an issue. Enemies that you can easily over power.
You can also get away with a damage setup instead of a shield if you are playing with other players. This creates a synergistic effect which can make the game a bit easier since you won’t be taking all the damage most likely.