One of the reasons to sign in is to roam your Visual Studio setting. Somehow what is important for me is that I can connect to TFS online everywhere I am (as long as you have Internet access).
1. When you newly start your Visual Studio 2013 or select FILE -> Account Settings… from pull down menu.
2. Type in your email account to login.
3. When you Select Team Projects… from Team Explorer then you will be able to access to TFS servers that you have connected before.