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
this.clusterService = clusterService;
        this.indexNameExpressionResolver = indexNameExpressionResolver;
        this.repositoriesService = repositoriesService;
        this.threadPool = transportService.getThreadPool();
        this.transportService = transportService;
        
        this.updateSnapshotStatusHandler = new UpdateSnapshotStatusAction(
            transportService,
            clusterService,
            threadPool,
            actionFilters,
            indexNameExpressionResolver
        );
        if (DiscoveryNode.isMasterNode(settings)) {
            
            clusterService.addLowPriorityApplier(this);
            maxConcurrentOperations = MAX_CONCURRENT_SNAPSHOT_OPERATIONS_SETTING.get(settings);
            clusterService.getClusterSettings()
                .addSettingsUpdateConsumer(MAX_CONCURRENT_SNAPSHOT_OPERATIONS_SETTING, i -> maxConcurrentOperations = i);
        }
        this.systemIndices = systemIndices;
        this.masterServiceTaskQueue = clusterService.createTaskQueue("snapshots-service", Priority.NORMAL, new SnapshotTaskExecutor());
    }
    /**
     * Same as {@link #createSnapshot(CreateSnapshotRequest, ActionListener)} but invokes its callback on completion of
     * the snapshot.
     *
     * @param request snapshot request
     * @param listener snapshot completion listener
     */