But then why do it in the first place? If you want to have the optionality to constantly plan, reassess, and communicate why would you bind yourself into a legal agreement that limits your ability to do that?
Because there are tangible legal and social benefits to doing so, including hospital visitation rights, insurance, etc, as well the ability to live in harmony with people you love whose social expectations are that you get married. This is important to many people—especially if they have found a partner that they can conceive of spending a lifetime with.