Just off the top of my head, jinglbells, I think you've hit it - it's mostly about financial matters. And maybe, also, the right to visit a very ill partner in the hospital - yunno, in the kind of situation where only a spouse or immediate family would be allowed to come in. But the people don't have to take vows and aren't legally required to be faithful or any of that. And I guess you don't have to go through a divorce if you decide to break up.
I guess some people would prefer it for those last couple of reasons. But I think I would expect a serious partner to be faithful, married or not and I'm sure many people do. And I've heard of couples still having problems over dividing property and so forth if they break up, which doesn't surprise me, TBH. So it may not be as different as some people think. But I could be wrong about all this.