One example:
In a nation wide (Netherlands) electronics chain the cashier asked for part of my zip code. The complete zip code in the Netherlands determin the city and which part of a street you live in. The first 4 digits determine the block you live in. The answer to the question of the cashier would give:
- the block I lived in
- what I bought
- At which time frame
- What kind of payment I used
- Probably by using a debit/creditcard who I am.
- The amount I spend.
- Which subsidiary of the store

Combine this information with thousands of other such transactions these stores can put together a lot of demographic information....