Answer
Small, critical, strongly consistent data that many nodes must agree on: leader election, locks, configuration, membership and shard assignments, with watches that notify clients when it changes. Not application data or high write rates: every write goes through one leader and a majority, and the whole dataset is meant to stay small.
Why
etcd limits requests to about 1.5 MB by default and suggests keeping the database under 8 GB; ZooKeeper caps a node's data at about 1 MB and keeps everything in memory. They coordinate the system; they are not its database.