Currently, changing ShardingTaskExecutorPoolMinSize requires a restart.
But safe settings require consideration of total router count. So, in clusters that use this setting to keep connection pools as warm as possible, adding or removing shards requires a restart of all routers.
Allowing ShardingTaskExecutorPoolMinSize to be configured via setParameter without restart allows shards to be added and removed without the added operational complexity and service interruption.
Note that the related setting, ShardingTaskExecutorPoolMaxSize, is not as coupled to router count as the min size is. But as these settings are commonly paired it probably makes sense to allow both to be changed without restart. That said, *MinSize is more important to be able to change on the fly.