the best use for comments in my opinion is if you can find popular sites in your niche, which have 1000s of visitors per day and you are allowed to comment (with a link) on new posts before other people comment. This way your comment stays at top and if it's good enough (engaging, annoying, insightful, or any combination of these) people will click on your link, giving you lots of free traffic in the long run.
Even if you're not the first commenter but in the first 10-15 comments made on that page it still matters as long as you are allowed to link back to your site.
This is when comments matter. SEO-wise they still carry some juice, albeit not as much as they used to carry 10 years ago. But the free traffic aspect is what you should be after, but.... it's not easy to find such sites nowadays that are heavily traffic-ed and which allow hyperlinked comments. I mean, it's not easy if you do it manually, because I'm sure that Scrapebox and other tools like that can find these websites, but whoever is doing this is probably keeping the discoveries for themselves