It’s a feature that many fans of the platisher (publorm?) have been begging the company for from day one. It’s landing in the wake of another recent update, which let you start writing an outline or short post in an editor located at the top of the page. Both changes encourage more writing by lowering the bar for how much work is “expected from” the person writing a post.
In fact, one of the use cases most frequently mentioned with regards to a theoretical (until this point) compose feature in the mobile app is that it could be used in place of the infamous “tweetstorms” plaguing Twitter since Marc Andreessen introduced them when he joined the service in early 2014.
Instead of manually stringing together a series of tweets to make a larger point, one could jump into Medium (which is already built on Twitter identities), put 150-250 words together, and quickly come back to Twitter with a beautifully formatted post.