Sacrificial Pit – how to use it correctly?

When I was playing through the campaign as Rai'lag in version 3.0, I spent three weeks just throwing all the hydras into the pit, and only then did I start adding them to my army. I think these creatures look better in a sacrificial pit. Although hydras are quite suitable for the role of cannon fodder in a warlock's army while he's casting Chaos. But this trick won't work online, as it requires a lot of resources.