Why is ParallelBatchWriterMode used when Applying Oplogs

In function OpTime SyncTail::multiApply, ParallelBatchWriterMode is used.
If primary is writing heavily, the latency of reading slaves will be very unstable. In our production environment, it may be 400 to 1000ms,
I know that ParallelBatchWriterMode is a global resource lock, which will block every read operation on every collections.
My question is that: why is it necessary to stop reads on collections other than oplog.rs?
Why can not a lock on local db satisfy the concurrency model?