The study of DNA is a great discovery for legal purposes because it opens up a powerful new way of identifying criminals in some cases. DNA evidence has been used both to convict criminals and to establish the innocence of people who have been convicted of crimes.
At times, the perpetrator of a crime leaves traces of his/her DNA at the scene of the crime. Police can preserve this DNA and use it to help identify the criminal. If a person is arrested for the crime, DNA can be taken from them and compared with the DNA from the crime scene. This can establish with a very high degree of certainty whether the person arrested was the one who was present at the crime scene.
DNA evidence can also be used in other ways. Perhaps the most obvious of these is in issues of paternity. DNA testing can determine for certain whether a certain man is the father of a particular child. This can be important in some civil lawsuits.
In these ways (among others), the study of DNA has been very important in the legal arena.