An incomplete shard snapshot will prevent a shard from moving away from a node (see `SnapshotInProgressAllocationDecider`). That means that when a shard snapshot completes it may become possible to perform some shard movements that previously were not possible, so we should trigger a reroute to start those movements. AFAICT we do not trigger such a reroute today.
server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java
UpdateIndexShardSnapshotStatusRequest,
        ActionResponse.Empty> {
        UpdateSnapshotStatusAction(
            TransportService transportService,
            ClusterService clusterService,
            ThreadPool threadPool,
            ActionFilters actionFilters,
            IndexNameExpressionResolver indexNameExpressionResolver
        ) {
            super(
                UPDATE_SNAPSHOT_STATUS_ACTION_NAME,
                false,
                transportService,
                clusterService,
                threadPool,
                actionFilters,
                UpdateIndexShardSnapshotStatusRequest::new,
                indexNameExpressionResolver,