For most, I think the increased stability appeals to some folks especially if they have balance/mobility issues. I imagine they could carry more overall weight because it’s distributed over 3 wheels rather than two.

Personally, I’d rather have a really nice cargo bike, but I kinda get the appeal of trikes.

Not sure about the wagon aspect: You should be able to connect a wagon to either a bike or a trike though, you’d just use different mounting systems or maybe different wagons. Eventually I want to get a wagon for my bike but I just spent a good chunk of change on fixing up my tires to be puncture resistant so that’ll have to wait.