It's cheaper in some situations (hard to say which without a design study), due to reducing shipping costs (as you note), maintenance costs, and cost to scale production up or down.

For "better", well, it puts the project within reach of a small team of inventors/hobbyists with minimal funding, and allows everyone who buys a self-replicating robot to own a bit of the means of production. The alternative, monolithic non-self-replicating factories, are usually owned by amoral narcissists who succeeded in a highly political company, and the technologies produced are typically licensed, not sold. I'd rather be an owner than a subletter.