The original purpose of a National Convention (Republican or Democrat) was to nominate a candidate for the party and to negotiate a party platform. This result may seem a foregone conclusion today, as the National Conventions we watch on TV are largely scripted affairs, funded by taxpayer dollars. So, in this election year, you may rightly ask, “What created this historical artifact and why should I care? What madness is this?” When a candidate wins a primary election in a state, all the delegates allotted to that state vote for him at the National Convention. If no primary candidate seeking the nomination has a majority of the delegates sent to the National Convention, then the purpose of the convention becomes to negotiate whom the party will pick to run in the election and what policies will appear in the party platform. Minority candidates in this circumstance can have substantial impact on the party platform in return for recommending that their delegates support a particular primary candidate. At past conventions (i.e. Democrats in Chicago in ’68) riots broke out inside and outside convention halls. The addition of limitless funds combined with scientific polling and campaigning techniques have reduced the current role of conventions to taxpayer funded partisan political advertisements for the two principle candidates, deliberately excluding all others (Libertarians, Greens, etc.). In the modern era (last 25 years), conventions are anticlimactic as incumbent candidates invariably go unchallenged by their party, and challenging candidates tend to focus their attention in the primary season on winning in particular states that have a large number of delegates to send to the convention. But during the primary season, once a candidate has lost one or two of these super states (states that carry a large number of delegates), he now finds it virtually impossible to raise funds to continue the campaign all the way to the convention. Hence, only one fund-raising survivor makes it to the National Convention. The historical influence of other viable contenders for the nomination is now virtually non-existent at a contemporary convention. Off the convention floor, National Conventions also bring all the politicos into one place; so all the lobbyists can have access at one time. During the week of a convention, hundreds of millions of dollars will be spent by corporations and special interests groups to entertain and court legislators and regulators, where they can gingerly explain their concerns regarding legislation affecting their interests. Hundreds of millions more will be promised to candidates for future election campaigns once casual accords are reached at these "parties." That is politics. After all, why do you think they called Democratic and Republican political associations “Parties” in the first place?

Basically the purpose of the DNC and the RNC is for the canidates for both parties, in this case Kerry and Bush, to accept their party's bid for the president of the United States. Another purpose for the convention is for both canidates to state their claim on why they would be the better president than the other.

Their purpose is to limit the national debate to two platforms and sideline any real popular voice.

they probably need to get together for some reason