Shift Sign-In (Steam/PC)

Okay, this also applies to my BL2 and BL3 on all platforms. What is with the Shift sign in, at the start of the games? I try to sign in with my Shift account name, Steam ID, etc…and all say User Already Exists. It will only allow me to add a name that is new to me. I don’t understand what this is supposed to be doing?