Why is it important to convince a potential criminal the punishment that they face is certain swift and severe

Deterrence

Explanation:

Any potential crime could have a potential victim or damage to a system. It is important that persons that are likely to commit a crime be told that it could not be possible to leave unscathed due to the severity of the law.

This is a tactic to discourage the potential law breakers from being involved in crimes that may even be violent and appeal to their self preservation.

In some cases when an individual commits a crime it is already at the point of desperation and they will behave as they will without bearing of a punishment.

If the law makers can properly convince these persons that it is not worth the satisfaction of committing the crime they may not proceed. However, in some criminal minds the punishment does not matter, the crime has been committed and it cannot be undone.

the answer should be arrogance & pride.