I will give you one small use case but generally stackoverflow is all about programming and this is off topic.
Nevertheless Document oriented Databases or NOSQL DBs are for example used if your data that you want to put into your DB have a different amount of columns. if you would put this data into an RDB you would end up with many empty fields and that might be undesirable. in a NOSQL DB you only store the fields that are necessary.
Maybe Wikipedia will help you too:
https://de.wikipedia.org/wiki/NoSQL
EDIT:
Actually there is a Stackoverflow question answering that:
NoSQL Use Case Scenarios or WHEN to use NoSQL