Lot of folks on here scared of marriage. These threads come up often. Let me get this.... Is somebody holding a gun to your head and forcing you to get married? Lol. If you don't want to get married then by all means don't do it. Its your life.
And if you have to ask...'why get married?' Then you are by no means ready to do so
But to answer your question:
To share your life, values, wealth, happiness, etc. With someone you truly love.
To start a family.
To commit yourself to your soul mate.
To be someone's support system, and to have a support system of your own.
To wake up to a ray of sunshine every morning and know they'll always be there everyday.
To take care of someone and have someone take care of you.
To have someone keep you grounded.
To be challenged intellectually (assuming you didn't marry a complete bimbo)
To have someone cook for you, do your laundry, take care of your child, lol.
To have a second family who you're as close to as your own.
Also doesn't hurt having a second income if your spouse also works (more toys for you).
Sure you could have all these things without getting married but if you're asking why, then I guess something's missing
Sure there are no guarantees that it'll work out but that's the case with everything in life you just have to do your best and take it day by day.
PS I've been happily married for 8 years with one baby and wouldn't change it for the world!