Because you need to prove you have insurance to drive and the proof is on your phone rather than a piece of paper. I don't know about America but in the UK, if

Because you need to prove you have insurance to drive and the proof is on your phone rather than a piece of paper. I don't know about America but in the UK, if you can't provide proof immediately the officer will give you a "producer", which is a legal notice requiring you to provide the evidence at a police station within some time limit (typically a week or two). Edited to add: I'd guess this is less common these days because the British police have access to a database of motor insurance anyway.