verbal vows are one thing - but the legalities of lawful marriage are what gives the government some legal teeth to back it up.The original vows define most of it quite well.
Anyone can say anything. And they may even mean it at the time. But people can and do change their mind.
The legal contract of marriage is protect people's rights while in the marriage and to formally pass down assets and properties to rightful heirs in the event of death.
And it is also to provide some legally binding structure in dividing up the assets and properties and insure the continued care and provisioning of minor children in the event one decides to end the relationship.
Right now there is kind of a big push in the manosphere and red pill community discouraging men from marrying because since women initiate 80% of divorces and in a divorce he would presumably lose half of the stuff.
But without marriage, someone could potentially take ALL of it while he was at work and his only legal recourse would be to file a theft report and hope the police and courts actually do something about it. At least with a lawful marriage the courts will have to follow procedure in dividing assets and support of the children through legal channels.
Legal marriage isn't really there for when people are happy and loving and things are going well. Marriage is there for when people get sick, die or split up.