Great question. PG years are typically offered at private high schools in the US, especially prep schools on the East coast. For example, I attended Deerfield Academy in MA and we had PG students on campus every year. The pros of a PG year is that it affords the student an additional opportunity to prepare for college - academically, socially and athletically. Often, PGs (as they're called) are athletes looking to develop and grow to enhance their recruitment potential. Some PGs, including athletes, benefit from the extra coursework to show colleges they are academically ready for the next step. Students are able to study higher level math, for example, and take more AP/Honors courses. Students also gain an extra year to mature socially and emotionally, which can be beneficial for young students who aren't quite ready to transition to college. And since PGs usually are boarding students, they gain that transitional experience of living away from home in the more structured environment of a private high school. This experience is invaluable for the upcoming transition to college. Finally, the PG year allows for more in depth commitment to your activities: another year playing in an orchestra, another year doing debate, etc. This extra time also allows for continued development as a leader - all things that colleges love to see in their applicants!
The only significant con with doing a PG year is that you are doing an extra year of high school. Most students are ready to be done with high school and be moving on to the college experience, so the thought of a 5th year of high school can be utterly unappealing. If this is the case, and the above pros don't make sense, then the PG year is not for you!
Here is a link to information about Deerfield's PG Year.