There have been cases where organiser wants to collect attendee data and for that an attendee form is attached and respective fields are added. This is normal and known to all of us.
However , in some cases, the organiser keep the right to add feedback or some additional point against each of the attendees but not known to the attendee and only known to the event team and its analyst only when the data is seen at the dashboard or it is downloaded in the CSV format. When you click to update the attendee data form the dashboard, that is where you are able to see this field.
In this case, you add a form field, and make it hidden. Once hidden, it is not seen to the attendee however it is seen during the time of Update attendee data.