Marriage is a contract between two people bound legally by the county, state, or country. The greatest benefits aside from the traditional "romantic" aspects are those that occur in the event one person becomes too ill to make their own decisions, dies, or the couple becomes divorced.
Essentially these rights ensure there will be some type of financial compensation in one form or another for at least one person. This may entail receiving a deceased spouse's social security and having all properties and valuables (automatically) revert to the surviving spouse.
Being married also allows you participation in health decisions should your spouse become unable to make them him or herself. Without being married you would have no legal standing if a family member of your mate stepped in unless there was a trust or other legal document drawn up before hand giving you power of attorney. Most "living together couples" don't have this as the majority of people tend not to think about long-term illness or dying.
Aside from these particular benefits every other thing can be had without getting married (ie: having children, living together, joint credit cards and bank accounts and so on.)
It's only if things fall apart or someone gets ill or dies that you truly see the (legal) benefits of marriage. Basically someone is "entitled" to get something! A state sanctioned prenuptial agreement!
If you have the bulk of the financial resources and leave your spouse he or she gets half and if you die they get it all. :-)