You give me way too much credit.
So, out of the box, MailChimp will let you get a permanently archived version of emails. I really hate the UX for it, and if you have e.g. user-specific links embedded (which I do for my emails sometimes, especially for e.g. sales or links that trigger auto-login), then the link gets published. That’s highly suboptimal for me.
So I thought I would do my own version of the email auto-archiving. Bonus points: it is trivially easy to do, since my Rails app generates the email HTML anyhow (I literally just copy/paste it into MailChimp since I hate their WYSIWYG interface), and it gives people easy, obvious share-this-with-others URLs.
Additional bonus: I edit all of my emails for the web version to include a paragraph 2 CTA to join my email list. This is, far and away, my largest source of newsletter growth.
As to whether to add new content to the email list or blog: I personally have two audiences in mind. Stuff for my blog is general interest to the tech community. Stuff for my email list is for people who have business interests in it, only. I typically don’t cross-publish between the two, but might make an exception in the future.