SQL Server Contained Database
A description of the feature including its benefits can be found here:
I would like it because it allows for managing users with password login at the database level instead of at the instance level. It is a feature that can be opted into on a per database basis once the capability is enabled for the instance. It would also mean I could develop locally and then deploy the database to your servers, including users.
My use case is having the ability to configure multiple users that can login with different permissions. A read only user, for example, or a user with unlimited rights to a specific schema while having only read only access to the rest of the database.