[Question] what are the differences between a full train set and 4 train sets hook together?

This is talking about the Seattle Link, but can apply to any light rail system. I’ve been to Seattle a few times, and I noticed that the Link will have 4 train sets linked together, instead of just 1 continuous train. I was wondering if there is an economical benefit to this. I feel like the capacity would be less than just having 6 passenger cars and 2 cab cars? I also feel like it would be more expensive to purchase 4 train sets to be linked rather than have 1 continuous train set? I know you can separate the train sets and split them up, so it would be cheaper to buy 1 train set at a time, but it seems less economical in the long run. Would love everyone’s viewpoint on this.