Once upon a time it was simply the status-quo for the bride to plan the wedding and the groom to simply be informed about when they are expected to show up. Thankfully, that trend is slowly changing.
Obviously, because the day includes both people, it should be a joint effort to pull it off. Beyond that, the planning of a wedding can serve as an invaluable indication of the types of the problems that you may encounter was the wedding is done and married life begins to set in.
Planning a wedding together is all about communication. Most divorces are, ultimately, due to a lack of communication. This exercise will help you figure out how to deal with it.
Doing this, you'll be able to determine how decisions are made. Is it a collaborative process? Do you each reach your own decision and then discuss them with each other? If so, you can explore if one of you always needs to be right...keeping in mind that this means the other one is always wrong. By right, I do not mean "correct," it simply means that one of you always gets their way.
Too often, this requires that the other person give in, meaning that they agree just to stave off an argument, not because they are really in agreement. This can easily lead to resentment down the road.
You'll discover if you have similar likes and dislikes. This is a tricky concept, as far as being a predicator of future success. Two people who always agree and have the same preferences can wind up feeling bored in the relationship. Two people with opposite preferences can end up fighting over them.
Also, by planning your wedding together, you'll have a chance to compare the way each of you tackles a project. Some people have a need to manage every, little, teeny, tiny detail while others focus on the big things and expect everything else to fall into place. While having one detail person and one big picture person together can usually be a positive thing because you will balance each other out. However, the different attitudes can also end up being a source of annoyance and disagreement.
That being said, having two detail people, or two big picture people, together is just as often a recipe for disaster. Two big picture people can often mean that nothing every gets accomplished because no one actually does anything. Two detail people can, believe it or not, mean nothing ever really gets accomplished also because there is so much focus on the details that the big picture is never realized.
As I said, the information learned during the wedding planning process is not a perfect indicator for the overall success of the relationship. It's simply a chance to get a good snapshot of each person so that you can be aware of the potential bumps and have a strategy for dealing with them.