It’s not to add your pass sanitaire to your booking, but it is to send in advance for verification all the documents required by the destination country (for some countries’ it can include PLF or other docs on top of vaccine certificate or test result). Once your documentation is cleared, you have indeed a mention « ready2fly » on your BP (paper or electronic) and it won’t be checked anymore at the counter or at the gate. At the moment’ it is used only on certain routes. KL has a similar system.