The sooner a crime is reported the easier it is for the police to investigate it. This can make it more likely for the person who sexually assaulted you to be charged and brought to court.
If you decide to report the sexual assault to police within three days of the sexual assault, a forensic medical examination may find evidence that will support what you are saying. In the case of a sexual assault by a stranger, an examination may also help to identify who the person is.