A benefit of LaTeX is that it outputs beautifully typset PDFs. ShareLaTeX and Overleaf accomplish this in the traditional way: you write LaTeX in a template and then compile a PDF using that template styling. Authorea in comparison separates format from content. Content is not restricted by a particular output format. At any time you can export a PDF Word or LaTeX version of your document in the style of your choice. For example your requirements may look like this: "download a one column line-numbered 12pt version with APA citations and figures positioned at the end of the document". Why is this important? If Journal X rejects your manuscript you won’t waste a day reformatting it for Journal Y. Authorea does it in one click. (And yes you can also easily export your LaTeX papers to Word's .docx format!)