They have no benefit for SEO because they're nofollow links, but have good benefit for traffic. The main thing is to let the viewers know where to click within the video, don't leave them guessing. That's a big mistake, at the end of each video let them know that they should click the link in the description which is below the video for more content.
You can even show them how to do it within the video, that will increase clicks, but never leave them guessing let them know where the link is. You can also use the description to link to your channel and other videos as well and if people stay on your channel to watch more videos then that's a good sign for youtube and might get you more free traffic.
But basically, the description is good for traffic and has no benefit for your website seo.