It is needed for the practical exam. This is done online through mijncbr.nl
It's a dozen questions of the type: do you have coronary heart disease, epilepsy, etc.?
If the candidate answered "nee" to all questions, an email arrives within 24 hours confirming that the candidate is fit to drive.
When any question is answered by "I", this request can be sent to the GP. This one can in turn send it to a specialist. It costs time. Therefore, I advise you to get down to it at the beginning of the course.
Note: if you have poor eyesight, but wear glasses and thus see normally, you can answer that your vision is good when asked.