OpenID

I am a beginner with Zotero. May I know :
(1) What is the purpose of logging into Zotero using OpenID?
(2) What are the characteristics of a valid and a good OpenID, which I am supposed to type into the box after item 4 below:

Login to Zotero
Register for a free account Forgot your password?
1. Username or Email
2. Password
3. Remember Me Keep me signed in for 2 weeks
4. OpenID [box for OpenID] To link your Zotero account with an OpenID,
enter it here and click Login with
5. Login with OpenID
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