On sharded environment, using early grouping, besides the use of an index, it would be nice that we be able to avoid the mongos regrouping process.
I'll try to explain that:
The real results(after mongos regroup) must looks like:
But, in some cases, mongos regrouping process is nonsense since the grouping key is same as sharding key. So, never got same group key from different shards.
So, the prior example, now looks like:
The real results must looks like:
So, the point is mongos regrouping process is a waste of time when you group using same key as sharding key.