So that it's easier to sample a number of items from each group, instead of writing lengthy DSL like this:

https://www.mongodb.com/community/forums/t/sample-x-number-of-documents-in-each-group-with-or-after-a-group-stage/170787

Ideally it can be expressed like this:


{
$group: {
_id: '$year',
samples: { $sample: 100 },
}
}


Hence making sampling 100 items from each group a snap to achieve!