When you enable approval routing on the journal entry, it enables a few fields, one of them being "approval status" and "next approver" which allows you to either create a workflow to set these fields or it follows out of the box approval which follows manager hierarchy.
The biggest difference is when approval routing is turned off, the approval is based on if the user has the permission of "approve journals". Any user with that permission can approve a JE, where as if the approval routing is on, the user must meet a condition to approve the JE (via native hierarchy or conditions set in a workflow).