Hell if I know.
I suppose if you want to have children with a woman it makes things simpler as far as parental rights and legal guardianship is concerned. Whether you want children or not is not the issue here, so I'm not going to digress.
I think that it's only because in the past (well, until about 100 years ago when some things started to change and not necessarily for the better) it kept a woman from being with a bunch of other men, and the man from ditching and screwing over the woman. The last part refers to the fact that in the old days, women were almost universally stay-at-home, while the man's income supported everyone within his family (the woman, and his children, on top of himself). In those days, if the man refused to support her, she'd be screwed unless she found another man, and quick, since working for money wasn't really an option for them back then.
Let's also consider technology, and it's effect on the issue. Even in the 30s (and I think 40s, but I'm not sure), and certainly before then, washing machines were rare so almost no one had them. Microwaves didn't exist either, nor did vacuum cleaners. Heck, in rural areas it was even common to assume that people didn't have electricity or phone service. Because of the lack of technology, it was harder to do the household chores, and they took much longer. It takes longer to sweep a floor than vacuum it, but they had to. It takes longer to prepare something and put it in the gas oven than it is to cook it in the microwave oven. Let's not forget the fact that without washing machines and drying machines, all clothing had to be washed by hand (takes a LONG time for lots of clothes), and then hung out to dry (longer than throwing them into the dryer). Also back then it was more common for the woman of the house to make clothing for the family, instead of buy it in stores, so that also took a lot of time.
I'm sure that I've overlooked a few things there, but you get the point. It was literally the practical thing to do, because of so much work that had to be done (since back then there was no technology to make it faster and easier).
So in the old days, marriage protected the woman. She'd get money because if she didn't she'd be out on her ass because women didn't work back then.
Now why is it that it's the woman who stays home, while it's the man who earns the money? At least back then?
Because males are the natural leaders. In the caveman days, he went out to get food for his family. Nowadays, it's not food you need to get. You need money (something that every established civilization that I know of, has), so you can buy things you need. By getting the money, you can provide for the family. Women did the house stuff for the sake of the man and the children, and back then were expected to. If you went back in time and asked someone about that, they'd look at you funny (since no one ever thought otherwise). When you finally get an answer, they'd probably either tell you "That's just how things are," or "The man makes the money, so the woman is doing her part pleasing the man since he supports her."
Now let's fast-forward to modern times:
The Feminist movement happened. Women now can work for money, they can legally hold any job that doesn't have a bona-fide requirement for maleness (and the laws are quite strict regarding what's bona-fide). It's not legal, for example, to say that men are natural leaders, so only men should be CEOs. Also technology improved. We have micrwave ovens, vacuum cleaners, electricity and phone service (even wireless!). Nowadays people buy their clothes in stores or order online. No one makes their clothes anymore. So now the huge burden that used to be on women is no longer there.
In short, a woman can now do something that even 70 years ago, would've been almost unheard of:
She can support herself independently, earning her own money, doing her own chores, and not need a man for anything.
This is all very good and well, especially because I'm cheap and I refuse to support a true dead-beat. However, there are downsides. Marriage laws have obviously not changed with the times. In divorce, without a pre-nuptual (sp) agreement the woman will get half your stuff, even though nowadays there's absolutely no reason for it (she can now earn her own money, and thus not need your money to live off of until she finds a new man).
Times have changed, and in some ways for the better. However marriage laws are very long overdue for change. They're screaming for change!
I will be damned to hell before I will marry without a prenuptual agreement, especially since I have a fairly reasonable chance of being quite financially capable and I won't want to give that up. Also because there's only one kind of person who I give lots of money, and that would be any child of mine I may have in the future (if I decide to have one). Plus, even if she's the rich one I still have my personal principles. If I don't want a woman I'm not going to take her money.
Given that I don't believe in heaven or hell, I guess I really do mean what I say up there.
Hopefully that provided some insight.
Ben