I keep forgetting to do to pin comments, and I'd love YouTube to implement one for creator and one for fan pinned comments, but currently you can use pinned comments to either highlight a fan who makes a super nice, or super entertaining comment that makes you and your community laugh, this shows your audience that you pay attention to comments and that you might even pin them, which can entice people to get more active in your comments, thus, boost engagement.
You can use the pinned comment to highlight an important feature in the video, maybe a link to a relevant video, some promotion or sponsor link you're doing, or a comment that highlights something relevant to the video or marketing that might be missed in the description "click here for merch" "discount code etc etc" "if you want to see more videos like this, check this video/check this playlist"
That's generally how I use them. If there's a bug in the recording, you can use the pinned comment to point out that there's a bug and not to worry about it, or if audio was bad, or something at the start of the video is a time limited thing, you can always do the good ol "video starts at ----" but with chapters now, the "video starts at" pinned comment is less important now.
--------
I'll also add, if you can keep everything "above the fold" (basically before the "see more") you can do multiple things, A link to more videos AND a call to action like "question of the day, what you do _________" or "what would you like to see in a future video?" or just ask some questions that gets engagement going AS WELL as a click.
