Short version
Ancient Spear / Rage Flip allows you to gather lots of mobs into huge packs. This is important so that when you manually cast Rends (which, unlike the auto-applied Rends from Ambo’s, benefit from Area Damage) they can hit as many targets as possible.
Long version
Usually, when players that are newer to the build hear “density” and “lots of it” they vastly underestimate how many mobs that means. We’re not talking 20-30. Sometimes we’re talking about an entire map’s worth, and then popping them all next to a pylon. If you want to see an example…
Around 5 minutes 30 seconds shows a nice open map, multiple elite packs, plenty of trash, and popping a Conduit pylon.