It's hard to prosecute sexual abuse of children: the witness is by definition suspect.
In a way, the prosecutor even represents the defendant: that person is part of the state.
Anyway, we need to decide how to use our power (i.e., discretion). We give prosecutors discretion because the facts are too variable to match a statute precisely. The statutes are too broad to give us clear guidance in all situation. We need someone to make an initial decision (which is ultimately a moral call) about whether or not *this* particular case should be prosecuted.
Maybe the prosecutor has a fiduciary duty to the state that includes educating it about some issues (i.e., sometimes prosecuting even when most citizens would have let the issue go, in order to send a message).