Yes it's a road worthy check, when your car gets defected you have to take it to regency to get the defect taken off, after you've fixed the problem (although you can also get like a lower defect where you just go to a relevant police station). haven't had much experience myself but if you get defected and you fix what you were defected for, they can look at other problems and defect you on other things.
Edit. And your car gets defected by a patrol car, like they pull you over for a reason then have a look at your car and defect you based on what they find.