Physicists were one of the first groups to see the advantage of sharing early versions of their articles--or pre-prints--with colleagues. They established arXiv.org as a pre-print server, and platforms from other disciplines have followed.  Disciplinary repositories allow you to share your work with others in your field without waiting forformal publication.  

As an aside, many communities are increasingly using Reddit, Twitter, and other social media platforms to share their work.  Bear in mind these less formal sharing tools may also be less discoverable and less appropriate for long term preservation than disciplinary repositories.