Barbara Jordan, an African-American congresswoman, gave a famous keynote speech at the Democratic National Convention in 1976. The first parties nominated their candidate in a meeting of party members in Congress, called a caucus . But when the American population started moving beyond the east coast, party members not living in Washington still wanted a voice in picking the party's nominee. The Anti-Masonic party set at date and a location, allowing party members from around the country to get together in one place and select the nominee. Now, people vote for delegates during primary elections and those delegate gather to pick a candidate.
Conventions can be raucous events.
Today's candidates know they'll be the party nominee long before the conventions even start. Not so long ago, delegates came to conventions unsure of who the nominee would be. Back then, delegates at the start of a campaign season would pledge themselves to a candidate, but by the convention could have a complete change of mind. This would make for highly-tense dramatic scenes . Delegates would cast their votes, but there often wasn't a clear majority.