Ref them having your mobile number.
If you have ever ordered anything from Amazon or ebay, and provided a mobile number for the courier, your mobile number will be with them as part of your record. If ever Royal Mail or Parcelforce have been the courier, they will also have a record. Big data is where records that have ever been put together, stay together on a new big data record. Part of big data is ensuring that only those who need the records have access. There will have to be strong control of direct sales. However, if you ever order centrifuges to process your nuclear fuels, the authorities will know about it.
As to why they do it, it can be automated and is a lot cheaper than leaving a (manual) card in your letter box if you are out, and then holding your stuff until you collect it. Reduced double handling, reduced storage area, reduced claims for lost or damaged parcels. It is also better service as you do not have to wait 2 days before you can drive down to the depot, struggle to park, and pick your stuff up.