If you set the Manager for a User object in Active directory, it creates a clear relationship between the user and their manager. This is reported in Exchange's Global Address List amongst other things.
For a Group object, the Manager is able to edit members of the group via the Address List in Outlook, if you enable it.
The same Manager field exists for computer objects in Active Directory. Does this relate to any functionality anywhere?
It doesn't inherently do anything, unless you have an application that acts on the data in some way... and I can't think of any application that cares about the manager attribute of computer objects.
It's there because both user objects and computer objects inherit from the same parent class, which is confusingly named User.
You could write an LDAP-enabled application that did something useful with the relationship between a computer object and its manager if you wanted to, though.