Real-time presence helps you see where your co-authors are working in the document so that you don't create conflicts as you edit. Additionally, the new ability to see changes to text and formatting as they happen will help you and your co-authors stay on the same page as your ideas develop and evolve.
And because Office Web Apps now save as you go, your team's edits are
included and seen as they happen.
Read the full announcement here: http://blogs.office.com/b/office365tech/archive/2013/11/06/collaboration-just-got-easier-real-time-co-authoring-now-available-in-microsoft-office-web-apps.aspx