In this code block what does requestAnimationFrame do?

  tick() {
    if (this.running) {
      this.remaining = Math.max(0, this.end - Date.now());
      requestAnimationFrame(() => this.tick());
    }
  }

In other words what would be the difference between the above code an calling this.tick() outside of the requestAnimationFrame():

tick() {
    if (this.running) {
      this.remaining = Math.max(0, this.end - Date.now());
      this.tick()
    }
}

For reference this code sample is from the What is Lit web component timer example.