A conditional function would allow a person building a form to hide or display additional fields based upon a response from a user filling out the form.  For example a parking registration form could require a student to identify each vehicle that he may take to school.  The student could select 3 from a drop down list to indicate the number of vehicle he may park at the school.  The form would display 4 additional fields(make, year, color, license plate) for each for all 3 cars (12 additional fields total)

This may sound familiar, as I added it about 3 or 4 years ago.  The idea was merged with another idea that was specific to Enrollment Management event registration forms.  Updates were made but only to the Enrollment Management portion of the suite.  It would be awesome to have this added to all forms.