Lots of reasons. I think it's romantic to stand up with my favorite person in the whole world, in front of all our friends and family, and promise to be there for each other forever. It's advantageous for career purposes because if one of us gets a job offer that requires a long-distance move, it makes it easier to negotiate at least some kind of position for the other partner. If one of us gets a job in another country, it allows us to immigrate together. It enables us to be on each other's health insurance policies and makes us legal next of kin for medical decisions. It provides legal protections for shared finances, makes it logistically easier to transfer money between accounts, and lots of other boring financial stuff involving IRAs/social security/survorship rights in inheritance/buying a house/etc. It entitles us to spousal privilege in a court of law, which hopefully never becomes an issue, but it's nice anyways. It makes things easier with my religious family, like we're allowed to sleep in the same room when we visit. It's an excuse to throw a big fun party. We want to have kids and it makes a lot of things easier in that regard.

Basically, we love each other, we want to share a life together, and marriage makes that easier in a lot of ways - financially, legally, culturally, and logistically.