In the document on Cursor Batches, it might be mentioned that setting a non-zero batchSize can be used to reduce the time needed for a cursor to do a getmore operation.
This can be effective when the performance difference between an initial query and a getmore operation is large.

More details on sharded environment - there can be wide variation in performance times between the intiial query, the first getmore operation, and then subsequent getmore operations. Using a non-zero batchSize will reduce the variation in performance times.