MongoDB remove() function deletes documents but retains indexes
The db.collection.remove() command is used to remove data from a collection. As such, documents will be removed from the collection and the indexes on the collection will be updated to no longer reference the removed documents. 

Remove can be used in such a way to only remove a subset of data. From the docs:

db.products.remove( { qty: { $gt: 20 } } )


For example, you may want to remove a subset of data but retain the existing indexes to continue access the remaining data according to the existing access patterns. 

This command contrasts from the db.collection.drop() command, which removes the entire collection along with its indexes.