From the Youtube page here:
YouTube suggests videos in search and recommendations that are likely to increase how much overall time a viewer spends watching and uses watch time to improve the ranking of videos in search and recommendations. If people click on your videos and stick around to watch, your videos are more likely to appear more often in suggested and recommended videos and growth in new views could increase. If your videos drive more watch time or inspire your viewers to check out a related video (either yours or someone else’s) your channel gets an advantage!
