Loyalty Card

I am a mother of 3 children. So if you think for yourself it can get quiet busy in a shop. So as a resolution I downloaded the StoreCard app to have all the cards on hand instead of having to walk with my purse having to fear it can get ****** whilst I'm trying to keep all my children together when I go to the shops alone. So today I decided to use some of my Loyalty points to pay for my goods. However I was then told that I need to have the actual card with me to use the points, upon asking the cashier and the customer consultant what the reason is for it no one can explain to me why....only answer I get is that is how it is. Why implement processes if you cant explain why? For me personally it seems like staff would need to go to training if they don't know why.