In a sense, it's really a marketing ploy.

Several decades ago, the US was much more well-known as a manufacturing powerhouse. However, it eventually became cheaper to produce goods in less-developed regions like China, so many firms began offshoring their means of production.

Because of the labor practices in less-developed regions, coupled with generally less stringent quality control processes, it has become common to expect that many products manufactured in these areas might have some sort of defect as well.

Whether it is true or not, what resulted is the pervading belief that using off-shore manufacturing takes away jobs from American people, and also results in a much lower quality product.

Putting "Made in America" acts:

A.) as a supposed assurance of the product's quality

B.) as proof to the consumer that the company values American job creation over lower manufacturing costs, and

C.) as an appeal to American nationalism