What is a good example of why you would want to restrict user or role permissions at the field level?

You may have a field that should only be editable by a manager.  An example would be market or region.  Another example might be promotion.

Some customers use integrations (e.g. webhooks) that may populate certain fields like review_rating that should be read only and only an administrator can override that value. Hence, field level permissions give you the flexibility to control what roles are allowed to edit it.