Registration Numbers

Can someone tell me why the club need to know the registration number of your car when booking?

I tow with either of two cars so dont always know which one I'm taking. I have sometimes used a different one from the one I've booked and they don't realise or ask.

Last week I booked a site for myself and 3 other units and the one thing they asked that I couldn't answer was the reg nos of their cars.

We always give our number, in the book, when checking in so I am puzzled.