When you need a rebalance
PCT's are most comonly used after male adults use hardcore anabolics like pro-hormones or similar.
After a hardcore anabolic cycle, your testosterone is high and estrogen is low. As youir body adjusts for not having all that testosterone, hormones may go crazy.
PCT may help to keep test/estrogen levels in check. This may help keep your gains!
Note: Recommended for adult males, and not for females.