I personally find the answers that have been given so far (a lap to lie in, emotional support, and so on) unconvincing. Why? Because you can have those things without marriage. They are good answers to "why is LOVE important," but they are not good answers to "why is MARRIAGE important." Love and marriage are not the same thing. There are many people who love each other but are not married, and many people who are married who do not love each other.
So I'll try to answer your question about marriage. What are the benefits of marriage? I think there are two kinds of benefits: social recognition and legal protection.
Social recognition is important to a lot of people. When something is important to us, we like to have social validation of that thing. Not all of those social recognitions are purely validation a; some of them, such as employer-subsidized insurance being available to the spouse of an employee, are economic.
Legal protection varies from place to place, but all societies have them. In the US, for example, married couples get tax benefits. They have medical power of attorney in the event their spouse is incapacitated and unable to do things like consent to medical treatment. Spouses can jointly own property (and there are certain protections on joint property in cases like civil lawsuits, which do not exist in non-marriage joint property ownership). A spouse can share in certain kinds of financial benefits such as pensions. There are legal and economic protections regarding children or other dependents. A person can not legally be compelled to testify against his or her spouse.