In more advanced systems (ala Mordor :-) ) now not only sum that you spend and place where you do it are stored indefinitely officially, but also full list of items you buy with any of your cards (or any of smartphone powered systems). It also concerns internet shops. Here full list is instantly transferred and stored in the goverment tax department. In less advanced places it is also done, but you will have special private aggregators (who collect anonymized data, of course!) who later provide info for goverment (in fully individual form where each line belongs to your specific person).

People who work with tax department know perfectly why they need it - full control of all expenses and also knowing of any items or things that goverment can deem as prohibited or unnecessary. Comparing your expeses with your taxes now exist in many countries already and is planned to become legally binding in next 2-3 years (can happen much faster during second wave of coronavirus).

If you buy any specific equipment (like big drones) or valuable things (like gold or silver). Goverment could confiscate them from you using new laws even 10 or more years from buying date.

Remember - capitalist goverment is not your friend, so keep everything from their sight.