Setting role-based security permissions on a transition is optional. The following are examples of when you would choose to allow or deny a transition based on one or more permissions for Journal Entry specifically.
Example: On the transition that connects the Submit node to the Approve node, you would select to allow the transition for users that have been granted the 'Approve Journal Entry' permission
Example: If you have users that should be able to Post a journal entry without it traversing the workflow (i.e. without going through approval workflow) then you could create a transition between the Submit node and the Post node, and select to allow the transition for users that have been granted the 'Post Journal Entry' permission.