What SEO value is there in blog commenting?

Blog comments are content on a page. Content on a page is indexed by Google. If a link is on a page, that link becomes part of Google's analysis of SEO. Therefore, you can leave a blog comment with your link and have that linked indexed by Google, thus counting as a benefit to your SEO.

Blog commenting, when done right, offers benefits beyond just SEO. ... If you add a link, it needs to be relevant and add value to the post

Blog commenting is the best tool for the link building

Blog Commenting is on of the best thing to do in off seo page