Suzy Khimm has a nice summary of what political scientists have to say about party platforms today, but she doesn't address a question I've seen a couple of times this week: why do parties have them, anyway?
Really, though, the question of why parties go to the bother of drafting documents which ordinary voters will never know about at all unless controversial planks are used in attack ads is actually a subset of a broader question: why do parties and candidates take positions on issues at all? Or, at least, why do they take positions on a broad spread of issues? To be sure: in some cases, there may be clear electoral advantage in supporting a public policy position. But usually, a candidate's best bet is to run on apple pie and the flag; why work at finding 60/40 issues when there are plenty of 99/1 "issues" out there?
The answer is that parties are not, in fact, just single-minded seekers of election. Instead, parties are made up of multiple individuals and groups: candidates, formal party officials and staff, campaign and governing professionals, activists, party-aligned interest groups and the partisan press. Some of these have obvious and clear interest in winning; that's especially true (at least in a normal party) of candidates and formal party officials and staff, and is also usually true of campaign and governing professionals. But others -- activists often, and party-aligned interest groups almost always -- may care far more about issue advocacy than in winning election. In some cases, the incentives may even make issue advocacy the main goal of some party actors, with the chances of actually changing public policy on that issue more or less irrelevant. It's also the case that many politicians and other party actors may have practical incentives for winning, but have been recruited from the ranks of those who case deeply about issues, and therefore have personal preferences for public policy which may override their institutional incentive for winning.
The outcome of all this is that parties do in fact take issue positions. How do we know -- how does the party itself know -- what those positions are? Well, there are lots of ways, but a quadrennial formal platform process at least gives one opportunity for issue positions to be fought out with a clear winner (or compromise) at the end of the day. It's not what it used to be when the delegates represented state and local party organizations and the national party only really existed during presidential election campaigns, but even with delegates who represent candidates (or, more properly, represent voters in their choice of candidates) it's still something.
The next step is for the party to find ways of binding their politicians to those positions in a system in which it's very difficult for a party to dump its incumbent politicians. Party platforms are useful for that, too. Not because they perfectly bind the presidential candidate to the party's positions, but because any constraint is better than none.
What's more, it's not just about the presidential candidate; there are thousands of politicians who run under a party's banner. A formally debated and adopted platform is one mechanism for educating all of those candidates about what the party, as a whole, wants to do. Of course, it's not binding on them, either, but again: it's something. Remember, sometimes it's not really about binding; sometimes there's a real lack of knowledge about public policy. A lot of candidates want to support party positions on the many issues where they have no particular reason to take a side but want to prove their party orthodoxy to their electorates. The platform is a way to do that.
Now, the platform isn't the only thing that constrains presidential candidates and other party politicians, obviously. Interest groups and activists certainly can and will attempt to solicit strong statements of support on their issues, platform or no platform. And a lot of this is purely tradition -- it's possible to imagine other ways of making collective party choices on issues and of constraining politicians, because we do in fact have those other ways. But those within the party who care about public policy and not just winning need all the help they can get, and so retaining the platform makes sense.