Markdown Editors can speed up the formatting process

Markdown is much easier to compose than HTML.

You can export in multiple formats

 The best Markdown editor will make it easy to write your content and quickly export it to PDF, web pages, or Rich Text Format.

You can work on cross-platform

Since there are dozens of really good Markdown editors for all platforms (Linux, Mac, Windows, web, etc.), you can pretty much write and edit text from any device and location.
This can bring a massive difference in your productivity.