In elections using voting systems where strategic nomination is a concern, primaries can be very important in preventing "clone" candidates that split their constituency's vote because of their similarities. Primaries allow political parties to select and unite behind one candidate.
In the United States, the small state of New Hampshire draws national attention every four years because it has the first U.S. presidential primary. (In 2004, the Washington DC primary had the distinction of being the first in the nation. However, it was non-binding and only 4 of the 9 candidates were listed on ballots.)
Other ways that parties may select their candidates include caucuses and conventionss.