Ability damage mechanic explained.

Hello guys, I've been wondering for a long time and now I'd like to get one thing straight:

How exactly does glyph/sigil /libram damage increase work? I know for example DKs have Sigil of the Vengeful Heart, which increases Death Coil damage by 380 and Frost strike by 113. Now how is this any good in comparison to some sigils that increase your Strenght for example by 200 for 20 seconds? There must be something that I'm missing in regard of how that damage adds up to the ability damage because my frost strikes deal like 5k damage rn and I'm with low gear, 113 dmg increase seems pathetic. This is just an example.


Thanks!

Frost Strike says:

Instantly strike the enemy, causing 55% weapon damage plus 138 as Frost damage.

That means that after equipping that sigil it turns into:

Instantly strike the enemy, causing 55% weapon damage plus 138+113 as Frost damage.

Thing is that "138+113" is a base that is increased by other stats (I'm not sure, which ones, I guess Attack Power?). In other words equipping that sigil does not simply add 113 points of damage to each Frost Strike.

This is added to the base damage of the ability. After this base damage, calculations with all the modifiers start to get the final actual damage.
I just searched it in Google, you can read more in the comments here: https://www.wowhead.com/item=45254/s...vengeful-heart