In one metro area I'm familiar with it is almost the norm in many circles to hold the boy back somewhere, girls not so much, and the earlier the better. Yes, it's all about athletics and most in the area I'm thinking try to do it early, like in elementary years. I'm more familiar with the Jr year HS "reclass" option (usually a boy transferring from public HS to private HS and repeating Jr year at new private school), or as RJM references the PG ("post grad" or 5th year of HS) year.
Son currently doing a PG year now at a school of 1100 total students. 25 PG boys (football, lacrosse, hockey, basketball mostly), 4 PG girls.
Benefits: if athlete still has projection remaining, allows them to grow and settle into frame and mature across the board.
Risks: Intellectually and academically? None, in my opinion. Recruiting wise? Can extend window of recruitment if, IF, athlete improves markedly from previous year. If nothing changes? No further physical growth or development? Most likely not a game changer.