A BLUES&TWOS card is a paper or plastic card containing critical information about the person carrying it in case of an emergency where the carrier is unable to convey that information urgently to a third party who may need it to help the person involved.
This could be an ambulance paramedic, a policeman, someone abroad, anyone looking to help in a situation where, for whatever reason, the bearer is at the time unable clearly to help themselves.
Most people are sensible enough to have smoke alarms fitted in their homes. Do they expect a fire? No. But the consequences of a fire that is not immediately extinguished are so catastrophic that few begrudge the cost involved.
The same logic applies to an emergency card. Hopefully it will never be used, but if it is, then that's the best £5 you ever spent. It doesn't even have to be a life or death situation, it may just be a toddler wandering off on the beach or in a shopping centre, a parent absent-mindedly tripping and getting into a state of shock or even a minor incident in a foreign country where you don't speak the language.
A call to a family member may avoid an otherwise inevitable trip to A&E, definitely to be avoided if at all possible.