Madness in the Bones

Hey everyone, i am building a Blood Mage with a focus on Physical.
I am looking for ways to increase my damage and stumbled across the Node "Madness in the Bones" which gives "Gain 6% of Physical Damage as extra Chaos Damage". THe Node is only accessible through a physical damage node, which gives "10% increased Damage".

What i don't understand is, why the tree is build like that and when one would like to pick that node. If my build is focused on physical, then i wont scale the gained chaos damage. If i scale chaos damage then i dont get anything from the physical damage. If i scale both then my build is likely not viable.

Am i overlooking something here?

Both chaos and physical damage contribute to poison magnitude, so there's that.

Chaos dmg is also just nice cause enemies don't have much chaos resistance.