Because at each appointment (even though it may be named the same, e.g. 30 minute subsequent visit) different procedures are done (e.g. laser + microcurrent or microcurrent + neuromuscular retraining or manipulation + laser, etc.)
It would be much more efficient and easier if the procedure codes the practitioner created in each note were pulled to the invoice.
This would prevent the additional step of having to manually add each procedure code to the invoice.
This would prevent the need to issue a superbill in addition to the invoice just to get itemization.