The test requires a stale router so any operation performed on a newly restarted shard triggers the behavior described on SERVER-32198. This way any fix of the PM-1645 project can be checked as they're committed. The sharding_csrs_continuous_config_stepdown test suite generates stepdowns, so when the stale router detects a change on the topology, among other operations, invalidates the database cache and refreshes the metadata, bringing it up to date. If this happens before any of the tested operations, it will make the operation to work as designed, and the assertions added to the test to detect the SERVER-32198 behavior will fail.