Honestly, this is not rocket science.
Here's my real-life scenario: To avoid data inconsistency, we have decided to change country/region type to dropdown, so there was no chance for the discrepancy when someone could potentially fill a country field with any other value than country.
Since, in most cases, inbound leads will present IP Country, I could copy the value from the field to our dropdown field Country/Region. If the value matches, then copy the value, if not, it's an error.
There are some other cases which we could benefit from, particularly cross-object cases as well I am not touching now.