The descriptions under your video it’s very important for your viewers and your videos YouTube SEO. You must plan your introduction when uploading the video to YouTube.
The description will help YouTube understand more about the videos. You don’t need to have an essay but a few sentences explaining the video and the sections within the video.
You can also link other videos in the series of the video to the description, so helping YouTube understand the topic and promote the other keywords/phrases it can rank for.
Viewers reading your description
You can use your description as an information portal for your viewers, for example, you can say “below is a link to such and such product”.
YouTube also always you to timestamp sections within your videos and you can list them in your description, therefore you can comment to your viewers, “I have listed below the different topics of the videos”, so you can skip to what you are interested in (Maybe you have long video). It’s better to have your viewers watching what they are interested in the first minute, so pointing to the topic can help them stay longer.