Probably out a desire for real love, to build a life with someone she loves, and to start a family. If not all three than at least the first two. Those are the reasons why I would get married.
Y'know...RPers tend to have unbalanced "beta" relationships before switching to TRP "alpha" relationships that are equally unbalanced in the opposite way. TRP as a whole doesn't have the best track record for successful relationships...why would they? If they did they wouldn't be TRP to begin with.
I'm guessing a lot more of TBP has had successful relationships. And it's hard to outline a "strategy" or a "cheat code" for such things because every relationship is extremely individuated. However there are some general principles that are good to follow. And they've already been outlined by multiple BPers in various threads over and over and over again. TRP likes to systematize what can't be systematized, and RPers refuse to accept advice that isn't otherwise painfully detailed down to every major decision.
Having a successful relationship changes you. You think about yourself and others differently. You grow up. It's not bad. Instead of wanting any sex one can get, or worrying about whether one is having sex "correctly", etc. You figure out what kind of sex you like, what kind of sex your partner likes, and how you both can get more of it. You figure out when your partner's feeling frisky and when they're not. You watch your partner's body change - and if you've based your love on something more than blatantly superficiality, you may find that you even like the changes. You just further develop into an adult with someone you love.
I intend to do a lot in my life and I'd like to do so surrounded by the people I love. I want to choose a partner who can inspire me with their own inherent passions for life. I do sort of want the captain/first mate dynamic that RPW talks about, but I'd prefer a relationship with such power dynamics within it (that go both ways, gracious!), rather than be defined by one constant, overarching power dynamic that continually imposes a certain social template upon the both of us in every situation whatsoever.
But do I want to be married one day? Yes. Yes I do. I want kids. I want a big, weird family and a lot of good friends loudly yammering around the kitchen table in a house I designed and built myself. I want the man I love beside me. That's the dream, anyway.
Would I consider any RP guys marriage material? Unfortunately no. If I had the opportunity to provide for a typical RPer, I doubt he would make a good homemaker, because I doubt he considers homemaking to be respectable work. He wouldn't take pride in maintaining our home or our kids all because I make the bacon. He'd resent me for financially emasculating them. It would be a disaster. I wouldn't touch such a prospect with a ten-foot pole.