There came the need to perform two operations, always, on 'application boot' time. Example? My real world example involved some geo-location logic (that's the first operation) and syncing of some session details so KcFinder can be in sync with the user in context - each user with his own session container (the second).
You have different collections in MongoDb with de-normalized attributes. As you may know, due to its not-join nature, MongoDb tends to encourage repetition of the same value in different collections (de-normalization). In this MongoDb is opposed to a more SQL-like approach where you usually reference only the ID of the value with a foreign key.