I like being married. Although if you were to ask me what's the difference between marriage and two individuals cohabiting in a monogamous relationship, legal and other benefits not withstanding, I cannot answer your question. I spent months after I got married wondering why I got married; six-and-a-half years after I have been happily married, I still cannot answer that question. Why did I get married? My grandmother believes that a person (men or women)'s final destination is to get married and have kids. Or maybe she feels then when her offspring/direct descendants get married and prosper somehow adds to her accomplishments in life. In any case, she was driving me insane by the constant bombardment of when was I going to get married; so I kind of just jumped into it. On hindsight, it was somewhat blindly. It was entirely luck that I jumped into marriage with the right guy.
In my defense, I never said that marriage is a necessary step in life, or the ultimate goal of a relationship, or that it's right for everyone. I am sure some people think the idea of going back to the same person every day is a nightmare. In fact, I have more respect for those who recognize that marriage is not for them than those who got married for whatever reasons and stay married miserably. That said, reasons I like being married (unionized/in a committed relationship, whatever float your boat) are:-
(1) I like to always have someone there, by my side
(2) I like going back to the same person every day
(3) I like going to bed with the same person every night
(5) I like having a partner in life
(6) I like being taken care of when I am sick
(7) I like to know that I have a build-in companion when it comes to social gatherings or work functions
The BEST of all reasons is the one supported by the saying "behind every successful man is a great woman" (the genders are totally interchangeable). I think it can loosely be translated into "Studies show that peeps in a steady relationship is more prone to becoming successful in life." (Sounds convincing when you start a statement with "studies show that", huh?) But why is that, you may ask? Think of the massive amount of energy saved when you are in a committed relationship. For starters, when someone of your opposite (or same, if you are gay) gender and within your age range walk into the room, you no longer have to gauge whether he/she is a potential partner; you no longer have to size him/her up to whatever criteria that you have; you don't have to worry about whether you meet his/her criteria; you don't have to spend your evenings/weekends searching/hunting for a potential mate; best of all, you no longer have to wonder about whether the other person is interested in you, regardless of whether you are interested; AND, once you are married, both parties (you and stranger) will tend to less misunderstand your words/actions as possible signals of interest. Now imagine focusing all those energy saved by being in a stable relationship into your JOB! Oh, and let's not forget emotional stability -- assuming you are in a reasonably happy relationship.
All in all, a marriage allow you to get in front of God/government authority/friends/family to declare that you are choosing this other person as your devoted partner in life. You are going from two individuals to one entity. An analogy will be a DVD shelf. Two singles tend to keep their DVD collections separate -- there's no need to waste time sorting when you have to get out on short notice. Marriage, on the other hand, is a shelf with both of your collections mixed in together -- it's a commitment that there will be no need for a quick and easy sorting.