It's important, but It's not make-or-break important.
I believe the YouTube search algorithm takes words from it and adds them as tags, but don't trust me on that one. Sometimes the description is shown alongside the video in some cases, and could potentially win over a new viewer. It's also useful for putting information in, such as Twitter address, Snapchat or Facebook etc.
Descriptions can help when you don't want to display something in your video. You can put things like social media links, or resources that you can update on the fly. Some sites say to use keywords in there, but I'm unsure if that really helps.
There are other more important things like a strong keyword driven title as well as saying the keywords in your video so that CC can pick those up. Not to mention tags.
ALWAYS have a description in your videos. Google and YouTube will pull tags from your description and help you to rank. The more tags you use in your description the better. Also, it is a good way to link other people to your social media and older videos.
Descriptions are as important as tags or more. The first line of your description will show up beside your video in the search results. Also, anything in your description will show up if a search includes those words the same as if they were all part of the title. It's like an extension of the title as far as search is concerned. That's why people can get Community Guidelines Strikes for listing a bunch of keywords in their description. That is what tags are for, but people put them in the description because the search algorithm directly pulls from the description.
Very important. It reinforces your tags. Try to get the tags in your description but not as tags. So if your tag is the game review put this is a game review bla bla bla. It will make that tag stronger. Same with title. And captions. They all help a lot. Placing high on search and suggested is by far the best way to grow properly on YouTube. You might see a lot of easy growth from promotion but something forced on you is never a better relationship than something you looked for and wanted.