Why would someone need a "get?"
If a person was married to someone of the opposite sex under Jewish law, that person cannot remarry under Jewish law without a get. Without a get, s/he would still be considered married under Jewish law, even if s/he received a divorce under state law (civil law). Also, without a get, children from any future marriages would be considered mamzerim (illegitimate) under Jewish law. You can also read What are “mamzerim?” How are they treated within Jewish communities? for more information. However, a woman who gets divorced under state law (civil law) can still legally remarry under state law without a get from her former husband.
Many Reform rabbis are willing to perform a wedding for a person who only has a civil divorce from his or her former spouse, but it is necessary to get a get to remarry in the Orthodox and Conservative communities. Reform, Reconstructionist, and unaffiliated Jews may still feel they need a get in order to conform to the strictest interpretation of the law.
Many non-religious Jews choose to marry under a chuppah (wedding canopy), sign a ketubah (marriage contract), and have a Jewish wedding performed by a rabbi or cantor. However, many non-religious Jews in heterosexual marriages may not realize that in order to remarry under Jewish law, they need to obtain a halachic (Jewish law) divorce. If you are a non-religious, unaffiliated, Reform, or Reconstructionist Jew who married in a Jewish wedding ceremony, you may wish to get a get so that you can remarry under Jewish law and have that marriage recognized by all denominations (streams) of Judaism. However, if this is not important to you, you may decide you do not need to pursue a get.
Orthodox authorities hold that Jews who were married in civil ceremonies must also receive a get in order to be able to remarry under Jewish law. If you were married under civil law, you may still want to get a get to conform with a strict interpretation of the law.