It is a formal expression of symbiotic commitment.
Although that word, "commitment" seems to have become interchangable with the word "marriage" itself. In my eyes marriage is the lovey-dovey happily ever after fairytale; Whereas commitment is the dedication to that fairytale once the harsh realities of interpersonal dynamics kick in.
Death is nature's agent of change. Right now you are new (or new-ish), but soon you, and your ideas, will be increasingly old, stagnant, and irrelevant. Then it will be the next generation's turn to give the world the dynamism that only youth and naivety can bring. Marriage is a complimentary solution to that inescapable ebb and flow of life. Both children and the acquisition of resources consume our most precious commodity - time. To invest in one is to compromise the other,that is of course, unless you have a commited life-partner to counterbalance your investment in either (Note - stereotypical gender roles not entrenched).