Well, right off the bat there is a proven advantage of a longer life for married couples.
But being married for now over 27 years I would list the advantages like this (please note I am referring to what would be considered a 'good' marriage as I think thats the context its offerred):
1. Stability. You have a lot less anxiety about uncertain issues.
2. Support. You have someone that will have your back no matter the issue.
3. Intimacy. There is actually a comfort to be found that someone knows you both inside and out. Plus, getting good nookie on a 2 to 3 times a week basis without having to work for it is nice too.
4. Companionship. Doing things together with someone you love is great fun.
5. Teamwork. Having a partner to help with various problems through life is a huge advantage.
And thats just a few right off the top of my head. If you wanted I could probably sit down and right an entire thesis on the advantages. But bear in mind such advantages can easily turn to disadvantages if your marriage is instead a bad marriage, and somehow things have gone wrong. But even then, I think the overall benefit is well worth the risk one takes.