Most Helpful Girl
Legally you need to get married so when you get kids custody and shit doesn't get all wack or whatever. Also if you're not married and you're paralyzed in a hospital they won't ask your mate whether a surgical procedure is the right choice, theyll ask the parents and some people dont want that
Most Helpful Guy
For some people it's a religious thing. For others it's just a cultural thing. (That's what people do when they love each other.") For others, there are many legal things that change when you get married. You can file joint tax returns which sometimes saves money on taxes. If you're married, even if you don't have a paid job, you can collect Social Security benefits based on your spouse's work record. At my job, I can include a woman I'm married to on my health insurance, but can't include a woman I live with but am not legally married to even though there's no difference in the relationship other than that "expensive piece of paper". You can receive unlimited gifts from your spouse without having to pay gift taxes and unlimited inheritance without paying inheritance taxes. (Of course those limits are so high now that very few people get that benefit.) There are downsides too though, like I think at least in some cases you can be held responsible for your spouse's debts.
Relationship-wise I personally don't see a lot of point to it, but there are a bunch of legal things that do change by getting married.