Chapter 5 limited parallel execution -> process.nextTick usage

[HowToDisappear]

Hello guys! I have a question about "spider" code logic that you have in Chapter 5 -> limited parallel execution. I don't understand the usage of process.nextTick(this.next.bind(this)). I mean I understand in principle what it does - it moves callback execution to the next event loop run. I also read Chapter 3 about unleashing Zalgo and it was easy to understand - you postponed code execution to timely set up event listeners. But in this particular case I don't understand why is it used and what could go wrong if we were calling this.next synchronously.

TaskQueue.js

export class TaskQueue {
  constructor (concurrency) {
    this.concurrency = concurrency
    this.running = 0
    this.queue = []
  }

  runTask (task) {
    return new Promise((resolve, reject) => {
      this.queue.push(() => {
        return task().then(resolve, reject)
      })
      process.nextTick(this.next.bind(this))
    })
  }

  next () {
    while (this.running < this.concurrency && this.queue.length) {
      const task = this.queue.shift()
      task().finally(() => {
        this.running--
        this.next()
      })
      this.running++
    }
  }
}

Spider.js

const spidering = new Set()
function spiderTask (url, nesting, queue) {
  if (spidering.has(url)) {
    return Promise.resolve()
  }
  spidering.add(url)

  const filename = urlToFilename(url)

  return queue
    .runTask(() => {
      return fsPromises.readFile(filename, 'utf8')
        .catch((err) => {
          if (err.code !== 'ENOENT') {
            throw err
          }

          // The file doesn't exist, so let’s download it
          return download(url, filename)
        })
    })
    .then(content => spiderLinks(url, content, nesting, queue))
}