What is the purpose of the database 'owner'?
There is some confusion out there between the database concepts of the 'dbo' (a user) and 'db_owner' (a fixed role) on one side and the instance concept of 'database owner' on the other side. The 'dbo' and 'db_owner' are often called 'database owner'. In what you're asking you are talking about the database owner as the server principal that owns the database. 

The theory goes like this: anything that can be granted permissions on is a 'securable'. All securables have an owner. The owner of a securable has absolute control over the securable and cannot be denied any priviledge. Instance level securables are owned by server principals (logins). Database level securables are owned by database principals (users). Principal come in two flavor: primary (identity) and secondary (membership). Server level securables are by default owned by the currently logged primary server principal. Database level securables are owned by default by the current database principal, except for schema bound objects that by default are owned by the schema owner. All securables support the AUTHORIZATION clause at create time to enforce a different owner. ALTER AUTHORIZATION can be later used to change the owner of any securable.

Since the database is a server level securable it follows that it will be, by default, owned by the primary principal that issued the CREATE DATABASE statement. Ie. the NT login of the departed employee.

So your question is really "Why do securables need an owner?". Because the owner is the root of trust. It is the owner that grants, denies and revokes permission on the object. Can a security system be designed without owners of securables? Probably yes, but there would have to be some mechanism in place to replace the role owners play in the current model. For instance consider that dad securables have no owner (eg. instead of owning a securable, the original creator is just granted CONTROL over it) it would be possible create a securable and revoke access on it to everybody, including himself. The requirement of an owner circumvents this problem since an owner cannot lock himself out.

The little known side effect of CREATE DATABASE of creating a securable (the database) owned by original NT login has burned many before. The rules are the same for every securable, but some factors aggravate the DATABASE owner issues:


the other server level securables (endpoint, server role, login) are far seldom used, moved around etc.
database level securables usually end up by being owned by dbo (the database principal), or some other database principal, and thus the owner is contained with the database
Having the database ownership default to the NT primary principal creates a containment issue (the owner is an NT SID managed by AD and does not travel with the database files, the NT account can be thumbstoned etc etc etc)
the most important thing: the database owner has important side effects, specifically the EXECUTE AS context. This later problem is what burns most users. Since Service Broker makes extensive use of EXECUTE AS (the message delivery has an implicit EXECUTE AS context, as well as queue activation that has an explicit one) is usually  Service Broker users that discover this problem first.


BTW, Kudos for investigating and fixing your original problem :)