Hint culture acts as a secret handshake that will reliably detect outsiders (they can't possibly learn the rules without identifying themselves as outsiders for a period of years). It will also help identify people who have less interest in helping others (because it costs more to recognize requests plus fulfill them and costs less to ignore requests). I don't think these are especially important reasons, but I mention them because I don't think anyone else has. These aspects would obviously have been more useful and less costly when mankind was more tribal, and less so with increasing globalization.