Question

What is the main purpose of a major party's national convention?

Answer 1

I'll assume that by major party, you mean the Democratic and Republican parties of the US. In that case, their conventions are mainly to select their candidate for the next President and Vice President and to set the party platform for the upcoming campaign following a primary. The Republican and Democratic national conventions are held once every four years as a result.

Other political parties, like the Libertarians, also have their own conventions but in the Libertarians' case, theirs are held once every two years instead of four to elect party officers and, in an election year, their own candidates for President and Vice President.