It's more todo with the Incarnate champion being terrible and Bone Widow being amazing. Bone widow is the only summon that doesn't have a massive hp penalty like other summons for some reason (example - Greater Incarnates have a 50% penalty, Oil Slug 60%), has a built in teleport move, and has really high base stats in general. It works well together since your summon level dictates the power of the bone spider and necro skills benefit from physical armour being broken which is the damage type the spider does.
At level 11 with 12 in summoning my Bone widow has around 210 damage, 2K hp and 300 armour as opposed to the pathetic 487 hp 148 armour 130 damage of my incarnate champion.
