baron: what does the strength added to zombies do?

I have not been able to find an answer to this.

What benefits does strength provide to zombies?

Same bonuses.

+1 to maximum Life per 2 Strength
1% increased Melee Physical Damage per 5 Strength

It used to be a WAY bigger deal back when stacking minion damage over 200 - 300% was a big deal

But these days you can pretty easily reach 600 - 800% minion damage with all the cluster jewels , timeless jewels , raw passive nodes , necromancer points and minion rolls on rings, gloves wands and fossil shields.