Aggregation operations process data records and return computed results. Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result.

By using the built-in aggregation operators available in MongoDB, we are able to do analytics on a cluster of servers we're already using without having to move the data to another platform, like Apache Spark or Hadoop. While those, and similar, platforms are fast, the data transfer from MongoDB to them can be slow and potentially expensive. By using the aggregation framework the work is done inside MongoDB and then the final results can be sent to the application typically resulting in a smaller amount of data being moved around. It also allows for the querying of the LIVE version of the data and not an older copy of data from a batch.

Source:

Introduction to the MongoDB Aggregation Framework | MongoDB. mongodb.com/developer/products/mongodb/intr...