Distributed's Adaptive's Scheduler `retire_workers` and Cluster `scale_down`

It appears that [Distributed's Adaptive tells the Scheduler to `retire_workers` and then `scale_down` the Cluster]( https://github.com/dask/distributed/blob/1.21.7/distributed/deploy/adaptive.py#L230-L235 ). Is there a reason both of these operations are needed? Also what does this mean for downstream libraries inheriting from Distributed's Adaptive or Distributed's Cluster?

xref: https://github.com/dask/dask-drmaa/issues/65 (for context)