DNA has enhanced law enforcement largely because it gives law enforcement more ways to identify who is guilty of a particular crime.
In the days before DNA evidence was widely usable, there was no way to identify who had left a blood stain or a hair or bodily fluids at the scene of a crime. Today, such things can be used to obtain the DNA of the person who left them. The police can then match that DNA to the DNA of everyone that they already have in their databases. If a person is not in their database, they can do various things to get their DNA, including surreptitious tactics such as picking up a cigarette that the person has smoked or a cup from which they have drunk.
Of course, DNA evidence can come back to haunt law enforcement. This occurs when such evidence proves that a person has been wrongfully accused or convicted. On the whole, though, it is a tool that makes it easier for law enforcement to identify possible suspects for any particular crime that they are investigating.