2. For LaTeX users. And for everyone else too!
The ArXiv relies heavily on LaTeX. For Authorea, LaTeX is one of many formats you can write in. Other formats include Markdown, and HTML (rich text, like Word!). Authorea offers a format-neutral, web-native platform. Regardless of the format, you render content on the web - not on a separate pane that compiles a PDF (side note, you can still export a PDF). The advantage: Authorea documents are not constrained to the hard sciences and to the LaTeX community. Authorea is for all scholars: from astrophysics to zoology. Authorea's rich text editor is a true WYSIWYG editor (What you see is what you get). It also has the ability to include as much LaTeX and mathematical notation as you need. Every document written in Authorea becomes a beautiful webpage, like this one.