Does this attribute increase damage done by summons?
If not, then why do people pick Featherfoot for Sorcerer?
Yes, it does. It boosts the damage of any summon that does Dark property damage.
but which one does that?
Dark
Skeleton Soldier
Skeleton Archer
Shoggoth
Ignas
Zaura
Netherbovine
Froster Lord
Gorkas
Templeshooter
Poison
Corpse Tower
Bone Pointing
Earth
Marnox
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It’s additive with gathercorpse/desmo though right?
Yes, it’s additive with those.
Wait…Addictive ? As in 840% + 50% ?
Not 840% x 1.5 ? 
That’s correct. It becomes 890%.