Even without issuing discipline, sometimes an accused or even an accuser will resign ahead of any investigation being completed. While this will obviously change how the investigation is conducted, it does not mean that an employer can get away without doing an investigation at all.
Moreover, an investigation will often uncover the systemic issues that may have allowed such an incident to occur in the first place. Aside from the conduct of any individuals, investigations will often reveal gaps in workplace policies, lapses in monitoring or security, or other issues that extend beyond the incident itself. Those will be revealed in the results of a thorough investigation, and can help lead to systemic changes that prevent future incidents.