Then what, exactly, are they in it for? What purpose on God's green earth does it serve? It looks like nothing except the perpetuation of beauracracy. How that is beautiful I have no idea. If you mean by "evolve", grow incessantly, I would agree.
It helps them to sort things out when things go wrong - when the marriage dissolves or a spouse gets sick or dies. There are rights and obligations you take on when entering a marriage. If you want the state to back up those rights, you register your marriage with them. Outside of that you are free to create whatever kind of relationships you want. Just don't expect the state to agree it's a marriage when your ass ends up in court.