The reality for employers is that they may not always be there to witness an incident of harassment. There is no flashing neon light that goes on every time an employee makes a crude joke or conducts themselves in a way that is inappropriate.
Employees must have a way to confidentially and discreetly report incidents of harassment so that employers can investigate as necessary. Employers are responsible for taking charge of the situation as soon as they become aware of a problem. In any workplace there should be multiple reporting options so that employees can report incidents without additional hassle and so that there is an alternate recourse in case the primary designated individual is also themselves the subject of a complaint.