ShareLaTeX and Overleaf bring the LaTeX desktop experience to the web. You compose LaTeX code in one window pane of the browser, press compile, and generate a PDF in a separate pane. While this writing paradigm is familiar to LaTeX writers (
a small percentage of the scholarly community), it is entirely unknown and difficult to grasp for non-LaTeX users (the vast majority of scholars). Authorea has an intuitive interface, similar to most other web applications.
Every document you write in Authorea becomes a beautiful webpage. Authorea is "web first".