While this Exchange server role shares the same name as a server role that existed in the last two Exchange Server releases, it is markedly different. In Exchange 2007, the Client Access server role provided authentication, proxy/redirection logic, and performed data rendering for the Internet protocol clients (Outlook Web App, EAS, EWS, IMAP and POP). In Exchange 2010, data rendering for MAPI was also moved to the Client Access server role.
Read more at source: http://blogs.technet.com/b/exchange/archive/2013/01/25/exchange-2013-client-access-server-role.aspx
