Why do the installation instructions say that the wiring harness must be approved or encoded?

Modern trailer control modules communicate with the vehicle’s control systems. Some functions, known as comfort functions such as light failure warnings, disabling parking assistance, trailer stabilising and rear fog light cut-off only work when the pulling vehicle has “learned” that a trailer coupling is installed. That’s why the trailer coupling has to be learned-in, approved or encoded, where possible using the vehicle manufacturer’s own diagnostic tester.