The workflow feature allows you to have different branching that can be very useful for filtering or targeting certain conditions or records so that the actions are executed in those. Some other time you might want to simply branch your workflow in case for example a contact replies to your email sequence and the further emails are not sent. For these cases you need to end the workflow in order to prevent confusion and have a clear flow.
Coming back to the example of the email sequence, assume you have two emails set up to be sent in the workflow with a break between the two of them. Then, you have another branch that would wait for their email and update a field in the record.
In both branches the stop workflow is missing. Thus when the first email is sent, and if the contact replies to you, the second email still would be sent as long as there is no stop workflow step at the bottom of the "Replies" branch and the record would not be completed:
Adding the stop workflow button at the end of two branches would prevent such situation and would allow the workflow to be completed for the records:
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