DNA is important to murder cases because DNA profiling can often help to identify the murderer.
DNA is often found at the scene of a murder. If the victim draws blood while struggling with the attacker, that DNA will be present. If the killer has also raped the victim, DNA may be present. If the killer has done so much as dropping a cigarette near the scene that he has been smoking, DNA will be present.
Of course, none of these things can actually identify who has killed the victim. However, these sources of DNA can identify people who were at the scene of the crime. This is, of course, of great help in a murder investigation and at the time of a trial.