Some rights can be similar, but you’ll always have to declare the other person as your legal whatever. Marriage says to the state that this person is my default for pretty much everything–power of attorney, medical stuff, property ownership, etc. So if I get in an accident and fall unconscious, my wife doesn’t have to fight the hospital staff to see me.

Depending on your country, there are other bonds that have the same legal binding as marriage.

In addition, if we’re honest, there are some “soft” benefits as well. My wife changed her name when we got married, and having the same last name (and our kids having the same last name) avoids a lot of complexity with things like traveling (especially because our daughter is a different skin color than the rest of us). Marriage didn’t explicitly grant us that privilege, but there are a lot of societal norms that come with it that have proven beneficial.

I’m not trying to claim that any of this is how it should be necessarily, but if you’re asking about practical reasons why, those are some of them. If you want the practical benefits without the cost, it’s (relatively) cheap to go to the courthouse or Vegas. Hell, you can get a friend to perform the ceremony for free, all you pay is for the marriage license. But if you’re otherwise not interested in marriage and those benefits don’t appeal (or whatever other reason), just stay dating.