“Marriage” gives you benefits and rights just living together won’t.
This is why we got married.
If your GF has an accident and falls into a coma, you have zero say in her treatment, as you’re just a roommate as far as the hospital is concerned. You can’t even go see her. Her parents will most likely make all the decisions, do you trust them?
Inheritance is another, depending on the local laws, usually the spouse has different rules for inheriting than a roommate, even if you have a separate will in place.
In here for example the other spouse gets to live in whatever apartment or house they were in even if the other half goes to the children or whoever is next in line.
Also I’m pretty sure banks give better terms to married couples vs two roommates buying a house.