Have configured the object cache many a times without really thinking about this - How does SharePoint 2010 uses the SuperReader / SuperUser(Object caching) accounts without password? Remember, there is no place to mention the password.
SharePoint doesn't need the password because its not actually authenticating anywhere with these accounts. SharePoint uses administrative and service accounts to fetch information and content from the the SharePoint databases. It checks its own internal access control lists (permissions) to determine the level of access an account has on an object - from no access to full control. If it impersonated the current user when accessing objects, each SharePoint user would need to have permissions to access the content databases (as an example).
When configuring the accounts you modify the SharePoint access control lists in the Policy for Web Application settings when you grant full control or full read. This gives the correct account the level of access to the objects for that site collection. I think the following information from Microsoft does a good job explaining why two accounts are needed and how the object cache works.
It also helps to keep in mind that SharePoint doesn't actually authenticate any users. It hands the credentials off to the authentication provider (Windows authentication, Forms-based authentication, or SAML token-based authentication) and the authentication provider tells SharePoint if the credentials are valid.
Administrative and service accounts require passwords because they are authenticating somewhere. For example, the Server farm account is used to access the content databases and therefore authenticates on your SQL Server.
SuperUser and SuperReader accounts are normal account not services accounts that's why these are not managed accounts.
SharePoint use these accounts to cache the data impersonating the App Pool Account. these accounts required Policy of web app level permissions and just grab the data from data base and put it into object cache. So when user make a request, instead going to database information grabbed from Object cache.
Read first 3 paragraph from this technet will explain you.