What is the importance of closed states?

Closed states come in handy in many situations.
You can:

• Prevent users from making changes to a record by closing it.
• Transfer open records from a deleted/suspended user to an active user.

Also, idle alerts are not generated on a closed record.
Closed records are not:

• Considered for auto-assignment with the Least Loaded assignment rule.
• Available for selection in a look-up field.