As PABaseball mentioned, health is a possible good reason for a PG year. I know a 2019 who will be doing a PG year at one of the elite prep schools in New England. He had an injury -- missed junior year and most of the summer between junior and senior year. He is a MIF with D1 measurables per recent public showcase data (sixty time in the 6.7s, infield velo in the upper 80s, exit velo in the low 90s) so I'm hopeful this path works out for him. Without the measurables, I think the chances of success are much lower.
Regarding MKS' follow-up question about California, per CIF rules, once you start high school your athletic eligibility ends after eight semesters, so there is no way to do a 5th year and participate in sports.