Answer
ZooKeeper deletes an ephemeral znode automatically when the session of the client that created it ends, which makes it a natural way to track live members or hold a lock. etcd does the same with keys attached to a lease.
Why
A client that crashes stops sending heartbeats, its session times out, and its node disappears; others watching it can take over. Adding a sequence number (ephemeral sequential nodes) gives each client a unique id or a place in a lock queue.