Use case

Manually run query against specific projection.
For https://kb.altinity.com/altinity-kb-queries-and-syntax/partial-projection-optimization/ it will allow to remove double GROUP BY from query.
Also it will allow to not run index scan against source table, which could take some time.

Describe the solution you'd like

SELECT key, sum(value) FROM database.table.projection_name GROUP BY key