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They do it because for many products, it works. It isn't easy to get a product on a store's shelves, particularly a new product with no sales history. And once it is there, most products don't really sell themselves. need advertising.

many times the maker of the product, and the producer of the video are in partnership. the producer of the video also often serves as the media buyer, and there is huge money in that.

infomercials are a very speculative business. they can make a ton of money if they "hit" or they can lose a ton of money. The air time is very expensive.

The way it generally works is someone invents or otherwise makes a product. they have nowhere to sell it, but they are willing to invest in production and advertising. they approach production companies to try to work a deal to produce and air the infomercial. if the production company likes the product and thinks it will work, they'll work out a deal, get the infomercial produced, buy the media, and get it aired.