I ONLY give other people cash, all my other purchases are debit/credit.
If you always use card payments whenever it's possible, it obviously isn't necessary to analyze your cash transactions to learn where you are because you are already disclosing it :)
Like MOST people and stores since Covid
There are close to 2 billion unbanked people in the world. In the US, it's less than 6% nationally, but over 10% in some states.
Many people who are not unbanked also often avoid electronic payments for privacy/security and other reasons.
The cash serial number tracking being described in this thread is useful for locating the neighborhoods frequented by someone who (a) avoids using electronic payments, and (b) maybe obtains cash from an ATM (or perhaps check-cashing service, in the case of an unbanked person) in places other than the neighborhoods they live in or frequent.
One example I mentioned in my comment you're replying to is check cashing services. Millions of people in the US receive money via things like check or money order and need to change it to cash despite not having a bank account to deposit it in; this usually involves identifying themselves.
See also payday loans, etc.
I assume you didn't click (and translate) the link in the comment prior to mine which you replied to?
If you do, from there you can find some industry news about Serial Number Reading (SNR) technology.
I don't know how widely deployed that technology is, but there is clear evidence that it does exist and is used for various purposes.