Primary elections are huge fund-raisers and advertising events for political parties. Holding a primary race increases the visibility of a party's candidates, and the constant news coverage and name repetition can make voters believe that the choices a party represents are really the only choices available. In the United States, parties that do not hold well-publicized primaries (third parties) traditionally get very small proportions of the popular vote.
During primary elections, members from the same party attack each other and bring to light negative things about one another. Then, after one candidate has been elected as the parties candidate, the opposing candidates in that party all of a sudden support the person they were just insulting. This makes those candidates then look like hypocrites and makes it hard to see any of the candidates as genuine. It forces people in the same parties to form grudges against each other and look dissented in the eyes of the public.