Because queries, specifically those that aggregate, consume memory and CPU on the live prod db. Something a simple scan cursor doesn't do. If the resource consumption is prohibitive, which it often is in mongo, and your use case is non-realtime, it's typically better to script the aggregation outside the DB query (or query an ETL'd aggregation store that doesn't impact customers when you lock it up)

Edit: changed "offline" to "non-realtime"