This is a great way to enable you to add customized fields to the Invoice document.

You can write your own custom DoInvFormRowCustomizedFields procedure, and the standard level0 code will call it.
This saves a lot of work when you update your customer to a new version, since you would have to merge any changes you did to the PrintInvoice procedure (for example), while you can leave your custom DoInvFormRowCustomizedFields as is.

I really hope they extend this functionality to other forms as well....custom fields are among the most frequent customization requests from my customers, and it really does save significant time when updating.