I occasionally still use paper tickets, since if you book train travel that requires changing London terminals (e.g. come in through Waterloo, leave from King's

I occasionally still use paper tickets, since if you book train travel that requires changing London terminals (e.g. come in through Waterloo, leave from King's Cross) your ticket is valid for the underground connection, and even if you book it digitally it'll give you a reference to collect a paper ticket rather than a QR code for (I assume) that reason. (Yes, I've made the mistake of forgetting/not realising to collect it!)