WordPress.org

Why would I want to allow multiple accounts to be associated with one email address?

Maybe your site is one that doesn't mind if users can sign up for multiple accounts from the same email address. More likely, you as an admin, plugin developer, and/or theme developer would like to be able to create multiple accounts on a blog to test various permissions or just want to test the blog having numerous users and don't want to have to assign unique emails for each account.

Can I limit who can create multiple accounts for an email?

Yes. You can specify a limit on how many accounts can be created for an email. You can also explicitly list the email addresses which are allowed to create multiple accounts (useful for just allowing admins to have multiple accounts).