What is the purpose of the $sort stage in MongoDB aggregation?

The purpose of the $sort stage in MongoDB aggregation is to organize documents in a specified order. When you apply the $sort stage, you're instructing MongoDB to rearrange the documents in the aggregation pipeline based on specified fields either in ascending or descending order. This is essential for operations where the order of documents matters, such as when you're preparing data for display or when you're computing further aggregations that depend on a particular sequence of documents.

Sorting can be done using any field in the documents, and you can also sort based on multiple fields simultaneously. The resulting ordered dataset is often necessary for reporting, analytics, and any use cases where the order of records defines the outcome of subsequent operations or visualizations.

The other options refer to different functionalities that do not align with the role of the $sort stage. For instance, calculating the sum pertains to the $group stage, merging collections relates to the $lookup stage, and identifying unique documents typically involves the $group or $distinct stages.