Say a diceGame system requires you to input a client seed before it decide what serverhash it will pair with the client id you provided. Is it mere stupidity to be suspicious the site doesn't let you give them the client seed while they show a hash of their serverseed?
they need to let you change your client seed anytime you want... and give you serverhash before and don't change it
in other cases, it can be manipulated by serverside