In the UK, the most important benefits are - sadly - to do with death. If you die intestate, then the presumption is that everything is left to your spouse. Similarly with your pension, a premature death would see the pension transfer automatically to the person you married. That's not always the case with partners.

You also benefit from spousal privilege, meaning you can't be compelled to testify against the other. Although, depending on your criminal inclinations, that may not be a strong reason to get married!