Actually a PG year is not (typically) just a 'repeat 12' at all. That would be a waste of time and money unless it was needed for academic reasons. Repeating 12th is another situation.
PG year is 'post grad' meaning, they have graduated high school and fulfilled all the HS requirements.
In this '13th' year, the student will often take the next levels of classes such as, taking calc if they stopped at Pre-Calc in HS, taking a more rigorous level of English, adding a social sciences or history class that had not taken in HS, the higher level of bio//chem / physics, etc.
On the other hand, a student who struggled in HS might do quite a bit or remedial / repeat work to better prepare him/her for the rigors of college.
The curricular choices depend on the school and the individual needs/goals of the student.
It offers a chance to re-take SATs as well.
Some students really benefit from the added year of social/emotional/ academic and physical growth. For some students the year at the boarding school is also a great transition.
I believe at some places you can also take classes that have college credit agreements.
The extra year can be very costly, of course!!