It's not why do you need it, it's that there's 2000 people that know how to do it with docker for every person that knows how to do it with cgroups. And your qu

It's not why do you need it, it's that there's 2000 people that know how to do it with docker for every person that knows how to do it with cgroups. And your question is probably just signaling you're in the second group (I'm in the first).