Restrictions are used as their name implies: to indicate specific things that a volunteer should not be doing, or negative attributes about them. "Skills" say what volunteers can do, now you have an easy tool to say what they can't do, and without having to explicitly lock people out of each and every job that they shouldn't be in. You can filter, sort and report on people based on this value, and this field can be used for non-volunteers if it's appropriate for your organization.
If there are no restrictions defined, the Restrictions field will not show in the Profile editor or in the properties of a job.