I have some experience with the PG route. I agree with a lot of what has been said, but have a slightly different opinion on some of it. There are really two different PG paths - the all baseball one like IMG and TCS, and the academic/athletic one. I look at it the same as a college decision - the old 4 years vs 40 years argument. High end academic prep schools are incredible places with extraordinary academic opportunities, and some have very good baseball programs, having kids move on to D1 every year.
That said, I don't think a PG year turns a D3 kid into a D1 kid without special circumstances. While I do think an additional year of maturity, both physical and mental is an advantage, I would tend to agree with the posters above who suggest it's not a game changer. I do think there is value if a kid has an illness that keeps him from playing (good luck D2020!), or maybe has arm surgery and needs to extend the recruiting process. High Academic prep schools aren't cheap, but I could not overstate what amazing places they are for young people.
I'm pretty familiar with some of the Northeastern schools that offer a PG year. Feel free to PM me if you have questions I may be able to help with.