You have a massive data set. You have geolocation data for the entire country. You are the king of GIS. And to prove it, you’ve developed a postcode lookup so people can check your database.
Your clients write their code and format their postcodes, and, like every other system they use, they expect that submitting a postcode will return an address. You’re the GIS expert, and you say the result should return Easting and Northing, because that’s what your database uses, and your database is right.
The clients look at the result, realise that your API returns garbage because it’s not using the language they expect, and move on to another provider.
Congratulations, less users to support.