I suppose with used cars most people won’t know if they’re buying until they’ve seen the car , and if it is a distance away , they will want to take it away there and then , rather than have another lengthy trip to collect .With a used car you can plug the registration number into CompareTheMarket (or similar) at any point, and get a whole bunch of quotes back straight away. Then if/when you want to go ahead with the purchase take out a policy with a future start date to match when you're picking the car up.
Of course if you're trading in you have the option to just change the car on your existing policy, as of the day you collect.