So i've put together a zdps build on a barb (hardcore seasonal).

my question is; why do people use ancient spear w/ rage flip?

it feels like i already have enough CC with stomp/charge freeze being up nearly %100 of the time because of cdr, why wouldnt i run something like wrath of the berserker for more toughness/speed or rend for another %10 flat damage from all sources?

if someone can clear this up for me that'd be great, thanks!

Ancient spear is used to grab monsters and put them close to your dps. you grab them from all over the map, and then group them together with ground stomp (or if there are monks in the team as support you just spear them and monk is doing the grouping).
Also, you do not need another toughness skill, as zbarb you just cannot die.

Ancient spear is (without exaggeration) THE MOST IMPORTANT skill for zdps barb.