The idea would be to allow selecting documents in midst a pipeline, remove them from the current flow, but return them together with the rest of the pipeline output.

Example case:
You have two collections with a ManyToMany "relation". Collection 1 has a boolean field and a list of ids from collection 2. 
Now imagine you would like to get all documents from collection 1 where the boolean is true or some property in one of the "related" documents from collection 2 meets a specific condition.
Right now you would have to do a lookup for all documents (including those with the boolean set to true). This could turn to be a inneficient if a lot of the documents from collection one have the boolean set to true, because it will still do a join with the other collection. And what if the condition for documents from collection 2 was complex and slow. Also what if there was a thrid collection with a ManyToMany to collection 2.

It would be easier and faster if you could just say:

{"skipAndReturn": <condition>}

as a pipeline step.