I must admit that I don't understand how Dietrich's Dragonslayer's Favor 2 in Dragonfall Director's Cut works.

Dragonslayer's Favor 2
Shaman AP 1 Cooldown 5

Concentrates mana from the surrounding area to provide an effect similar to a medium leyline. Lasts for 3 RNDS.

Mage (ingame help page)
Moderate Leylines:
+10% Accuracy to Mage Spells
+1 AP Damage to Attack Spells that cause AP damage.
+2 HP Damage to Attack Spells that cause HP damage.

It creates a leyline where you might want one, such as behind a good cover position. Should boost attack spells cast by Dietrich or any spirits he summons, or maybe Dante, or your own character if you're playing one that casts spells (including the bolts that counts as weapons). Mostly for mages I guess, but maybe some conjuring spells might benefit as well?

I don't think I ever bothered using that ability, as I always had more beneficial things for Dietrich to do, such as buffing and/or moving or throwing knives to flush enemies out of cover. I also played a rigger and not a mage, so that surely affected my decisions in this regard. Had I played a mage I probably would've used this ability a lot to avoid having to stand out in the middle where leylines were usually placed.

The accuracy buff is the biggest help, situationally useful for some tough fights. It also can make AP damage spells useful enough to actually choose over HP damage spells.