The main use case for these methods is to ensure that applications can read their own unacknowledged writes, but this is no longer so prevalent since the driver started defaulting to acknowledged writes. The other main use case is to ensure that related read operations are all routed to the same server when using a non-primary read preference. But this is dangerous because mongos no longer provides this guarantee. For these reasons, this method is now deprecated and will be removed in the next major release.