The party platform is suppose to present the aspiration and goals of the party if elected. The first platfrom presented was an address to the people that the 1835 democratic convention approved.
At the conventions there is often disagreements between the "true believers" of the party who wish the platform to represent their views, while the more moderate members of the party and often those people who represent the soon to be nominated Presidential candidate wish to moderate the platform so it attracts the swing voter. If the parties nominee has close control of the party itself he will work to insure that the fight over the platform is not too devisive and does not obscure the message of the candidate