The NI Central Postcode Directory provides a 'lookup' between active and terminated postcodes and statistical and administrative areas (i.e. for each postcode, which areas it falls under), and coordinates for each postcode.
The CPD is useful for spatial analysis of data that includes postcodes, and for linking postcodes to areas (wards, councils etc). It is unlikely that it has much commercial use, but could be used to help identify where customers are (though Googling postcodes will provide the same result). It is a very useful to research, in particular aggregating to areas as it provides geographic consistency.
However, the licencing and availability of the CPD is confusing and limiting. The restrictions of the current licence also limit what users can do with the data, especially when it comes to incorporating it into their own datasets.
I will explain more about the confusion and limits in comments due to the character restriction here.