When you are building distributed systems often you need a way to coordinate between nodes. You can use a single node to do it, but then you have a single point of failure.

In case of Zookeeper you can utilize various recipes, like here: http://curator.apache.org/curator-recipes/index.html

You can even build more advanced systems on top of it:

https://bookkeeper.apache.org/index.html

https://cwiki.apache.org/confluence/display/BOOKKEEPER/HedWi...