Commonly, not having a list is said to be ‘leaving money on the table’, and that much is certainly true.

Take the example of a conventional clothes store. Customers come, browse around, maybe buy something, and then leave. Once they’ve left, the store has no way of getting in touch with them, and maybe convincing them to come back again.

On the other hand, if the store had their contact details, they could notify them when they have a new stock of clothes on the...