This tutorial has been updated after Pro v1.10.0 release to account for improvements to the booking form API.
It some cases it might not be desirable or convenient to add fields via the form customiser. This may be because you as the developer do no want the actual end user to be able to edit or remove a particular field. Or you may want the field to have a known ID (which you set) rather than a numeric one allocated to it by the plug-in.
In any case, if using the form customiser is not suitable for your particular requirements, then you can always use the API. This tutorial shows you how.