Are national party conventions in the United States important to electoral politics because they empower party leaders or because they control who delegates vote for? Or is there a different reason?
In this day an age, the two major party conventions in the United States only serve one purpose. They are a week long infomercial where the national media focuses on one party, then the other.
So, then, why would the press give the two parties this advantage? The answer is history. Prior to the adoption of the party-wide referenda that is the primary election, the caucus really was how the candidate was selected. (Simple translation - a primary is an election open to all members of the party, a caucus is just a select few.) The "smoke filled back room" really was just that - it was the place where the people who showed up really had the power to select a candidate.
Absent national media, it really was possible to choose whoever you wanted. Harding, for example, in particular, was a candidate selected by the Republican Convention for electability. Garfield was able to subvert a Grant third term. All of this was made possible by delegates who had real authority to select whomever they chose to go head-to-head with the opposing candidate in the general election.
Even with the advent of radio and TV, floor fights could take on real serious consequences. But then, 1968 happened. In a year of tumultuous violence (in which one of the leading contenders - RFK was killed), the DNC Convention was a disaster. A floor fight turned into a real riot. Protests drowned out the actual work of the convention. And, to top it all off, Muskie wasn't able to make his speech until the wee hours of the morning. Anybody looking at that fiasco knew that the process was unpalatable to the voters - and when Nixon crushed Muskie, the Democrats decided not to repeat it. The reforms enacted in time for the 72 convention ensured that the real work of the convention would already have been worked out - away from the limelight of the cameras. This meant that the message could be honed. The Republicans followed suit, and realized that the convention could be a showcase - if party disagreements were held in check. The result? The week long infomercial you have today.
Historically, the convention really was the powerhouse. And, technically, if the rules were relaxed, they could be again. But the traditional coverage is now just that traditional. Networks have scaled back their coverage, but there is a disncentive to ignoring the whole spectacle, so it still exists.
But trust me, the excitement is long gone.