A good reason: If it's culturally expected and universal in the country you live in.

Or even in expected in another country, e.g. I almost married my Turkish girlfriend a few years ago. (The relationship ended before that happened, very long story) I'm in Australia, but that really felt like we were in Turkey. Couldn't walk down the street holding hands, because Turkish people would see and the family would find out. It seemed having a boyfriend is a kind of shameful, bad thing for a woman, but suddenly if you get married, it's all wonderful, you're no longer fair game for harrassment etc. I've never wanted to get married in the least (it never came up with any girlfriend I'd had before that), and don't know how anyone can promise to love another forever - but to make my partner and her daughter's lives better, in many ways, so easily, seemed a very good reason.

To raise children, to have regular access to safe and intimate sex, and all the lifestyle, cost saving, and health benefits of cohabitation with someone, in the expanded pool of potential partners that includes women who want marriage. But opting out of family life and ending your gene line is fine, it increases competition for the men who remain.

Not really, the family courts in most Western nations have made it completely untenable.

It saves inheritance tax.

If you're married a lot of things are easier if one of you passes away, otherwise things have to be prearranged. Are all household bills in joint names, are all investments with named beneficiaries, are all assets defined in a will, are all credit agreements covered, are there end of life choices documented?

Marriage is a formal agreement between two parties with respect to the rules through which they will break up.

I thought it still had some tax advantages.

The lawyers benefit from it at least.

Otherwise it's a lose / lose situation for everyone.

Because you love them and want to express that?