Originally, the main purpose of political conventions was to nominate the party's candidate for president. In the 1800s, the movement in the United States was to place more political power directly in the hands of the citizens. Political conventions were one way of doing this: Previously, candidates were nominated in secret caucuses by members of Congress; candidates would now be chosen by delegates who were selected at the state or county level by the party members.
The democratization of presidential elections eventually took the nominating function away from the conventions. People wanted more direct control over their party's nominees, so the presidential primaries came into use (although some states still use caucuses). Party members vote in the primaries to choose whom they want to represent their party in the upcoming election. By the time of the convention, there is no suspense about who will be the nominee -- it has been known for months.
Political conventions serve other purposes beyond nominating the party candidate, which is why they're still around. The convention offers party members a chance to gather together and discuss the party's platform. The platform is the party's stance on the political issues of the day. For a long time, the convention was a place for political debate, and important decisions were made there. In 1860, the Democratic Party debated the government's right to outlaw slavery. When the party adopted the position that the Supreme Court could decide the slavery issue, delegates from several southern states walked out, resulting in the Southern Democratic Party. In 1980, Senator Edward Kennedy fought against incumbent Jimmy Carter's economic plan. Although Carter defeated Kennedy for the nomination, Kennedy's debate forced Carter to radically change his plan.
Today, even this function of the convention has been largely stripped away. The conventions have been streamlined, with important events and speeches scheduled for prime-time television hours. The parties work to eliminate any evidence of debate or disunity within the party. The political conventions have now been reduced to the status of infomercials, marketing the ideas and personalities of the party to the public. While the conventions serve to unify the party and generate party pride, the "advertisement for the party" has become the primary function of political conventions today.
Now we'll look at who gets to go to the conventions, and what they do there.