Here is an easy example to understand the utility of the dislike count:

Imagine you are looking for a How-To video on something very common but specific, such as changing a car part or Windows setting, and that when you search for this you are given dozens of options. All of these options are 20 to 30 minutes long and 90% of them are either inaccurate, out-dated, or automated click-bait. This lower 90% will have a disproportionate number of dislikes. Each time you click one of these videos the dislike ratio can save 20+ minutes of your life and this benefit is multiplied by dozens of videos and tens of thousands of viewers.