@NerdyPotato Because that's what marriage is ...two people that stand up before a court or in front of a minister,in front of witness,to say they're going to be loyal to each other.(vows) It is a celebration of that commitment and why there are witness to the ceremony.
Is everyone's personal choice whether they want to make it official or not, but I see no reason to make it official if you're not going to be exclusive .
@Justmeraeagain there is no need to defend it. I think marriage is more than respected and even expected by society. I personally think you can commit to someone just fine without a wedding, and that placing a signature in front of an official and witnesses doesn't make the commitment or love any better or more meaningful, but celebrating that decision makes sense. And even if it wouldn't, it is your decision and it doesn't matter whether I understand or agree with your motivation. I was just curious, that's all.