Full repeal of the Defense of Marriage Act, which the platform language calls for, also means that all states have to recognize same-sex marriages contracted in other states. It puts the party in opposition to letting states decide the issue, which is the current rhetorical posture of the president.
UPDATE: Let me develop this rather hastily-made argument a bit further. DOMA has two main parts. One says that the federal government will treat marriage, in its own laws and programs, as consisting of the union of a man and a woman. A second says that states may refuse to recognize other states’ same-sex marriages. The idea was that even if a court rules that states are obliged to give full faith and credit to other states’ marriages under the Constitution, states that resisted doing this could take advantage of the constitutional language subjecting that full-faith requirement to congressional regulation.
Some Democrats have in the past called for repealing the first component of the law. It seems to me that there is no point to repealing both of them except to make it harder for a state to maintain a law that defines marriage as the union of a man and a woman.