请问 computeExpirationForFiber 中 var expirationTime; 之前的代码起到什么作用？

Question

function computeExpirationForFiber(currentTime, fiber, suspenseConfig) {
  var mode = fiber.mode;

  if ((mode & BlockingMode) === NoMode) {
    return Sync;
  }

  var priorityLevel = getCurrentPriorityLevel();

  if ((mode & ConcurrentMode) === NoMode) {
    return priorityLevel === ImmediatePriority ? Sync : Batched;
  }

  if ((executionContext & RenderContext) !== NoContext) {
    // Use whatever time we're already rendering
    // TODO: Should there be a way to opt out, like with `runWithPriority`?
    return renderExpirationTime;
  }

  var expirationTime;

  if (suspenseConfig !== null) {
    // Compute an expiration time based on the Suspense timeout.
    expirationTime = computeSuspenseExpiration(currentTime, suspenseConfig.timeoutMs | 0 || LOW_PRIORITY_EXPIRATION);
  } else {
    // Compute an expiration time based on the Scheduler priority.
    switch (priorityLevel) {
      case ImmediatePriority:
        expirationTime = Sync;
        break;

      case UserBlockingPriority$2:
        // TODO: Rename this to computeUserBlockingExpiration
        expirationTime = computeInteractiveExpiration(currentTime);
        break;

      case NormalPriority:
      case LowPriority:
        // TODO: Handle LowPriority
        // TODO: Rename this to... something better.
        expirationTime = computeAsyncExpiration(currentTime);
        break;

      case IdlePriority:
        expirationTime = Idle;
        break;

      default:
        {
          {
            throw Error("Expected a valid priority level");
          }
        }

    }
  } // If we're in the middle of rendering a tree, do not update at the same
  // expiration time that is already rendering.
  // TODO: We shouldn't have to do this if the update is on a different root.
  // Refactor computeExpirationForFiber + scheduleUpdate so we have access to
  // the root when we check for this condition.


  if (workInProgressRoot !== null && expirationTime === renderExpirationTime) {
    // This is a trick to move this update into a separate batch
    expirationTime -= 1;
  }

  return expirationTime;
}

请问 computeExpirationForFiber 中 var expirationTime; 之前的代码以及最后的if条件起到什么作用？

if (workInProgressRoot !== null && expirationTime === renderExpirationTime) {
    // This is a trick to move this update into a separate batch
    expirationTime -= 1;
  }