As Bailey described, a good example of having RS without PC for traffic violations is when you initially pull someone over for DUI. If they are swerving, driving under the speed limit, etc; that is RS. You do not yet know if they are really drunk, but have a reasonable suspicion that they are. Probable Cause could be obtained by talking to the driver and seeing that his eyes are bloodshot, reeks of alcohol, fails the field sobriety test, and so forth. With PC you'd be able to arrest the person for DUI.
Routine checks are illegal because you cannot be detained in any sort of way unless you're suspected of a crime/violation.