Do marks of chaos affect monsters with companion keyword?

I'm looking at making a despoiler list for some monster Mash fun, and I'm noticing a lot of monsters already have keywords like undivided, khorne, etc. For undivided and khorne specifically, can they actually benefit the unit's attacks at all if they have the companion keyword? I'm wanting to play a Slaughterbrute which has khorne mark and keyword by default but is it there just for flavor? Or does it actually help?