Most obviously, it’s helpful for subscribers who can't view the HTML version of your email or download images in their email programs.
The ‘read online’ link provides a permanent url that you and your fans can use to share your newsletter, now and in the future. You can use this url to link back to your newsletter issue from your website and blog.
This link is also a fallback option to social sharing links: readers can copy/paste the url to share on any social media platform regardless of what sharing buttons you include or don't.
I receive a weekly newsletter from ColourLovers called Happy Huesday. It’s a wonderful newsletter filled with links to colourful, interesting articles. Every week I want to share it… and can’t. They don’t include social sharing buttons OR a read online link. What a wasted opportunity!