I’m always surprised to hear this as a passenger boards the bus but not one day goes by without at least one person asking it. My reply is usually along the lines of “No one gets on my bus without a ticket, of course I need to see it!”. It made me wonder if there was any other form of transport which didn’t require the traveller to provide any proof of payment/entitlement to travel. Certainly not local bus services, nor airline flights. Trains? I can’t recall the last time I travelled by train so I’m not sure. Can you travel on a train without showing a ticket? I doubt it.