We are mighty proud of our REST API. It is — without editorializing — the nicest API for managing newsletters and subscribers on the market, thanks to lots of time and energy. Which is why we charge for it! There's nothing that you can only do through the API, but it's a great way to automate your newsletter and subscriber management and enable power use-cases.
One use-case that isn't quite "power" and is more "table stakes" is the ability to create subscribers through the API. This is something you can already do programmatically regardless of your payment plan by hitting our form endpoint, and so when folks pointed out that this was a bit of a weird restriction, I hemmed and hawed and figured — eh, why not?
You can now POST to /v1/subscribers and create new subscribers through the API, even if you're on a free plan. Check out the docs to get started!