The reason it is important that sexual assaults be reported and investigated in a timely manner is due to the fragility and time sensitive nature of the evidence.  Evidence of sexual assault includes bodily fluids, hairs, fibers, and also cuts, lesions, and bruises on the skin.  The more time passes after the actual assault, the DNA based evidence of the fluids, hairs, and fibers will be washed away, diluted, or otherwise destroyed by natural processes.  Also, the skin based evidence such as bruises or cuts will heal over time.  So it is extremely important that cases of sexual assault be reported immediately so the evidence can be obtained and documented before it is too late.