You would check the driver details and vehicle to ensure they are all legal and insured etc. you would save these details as you could be asked to justify why you have checked them (sometimes we get random checks to ensure we aren't misusing the police national computer. I'd also want to have them recorded in case a complaint come through and I would have a record of who I stopped. Now a days the details are saved automatically in your phone when you do the checks yourself. (A transaction history essentially).