You know you have a good invention that is useful and fills a need based on your own experience. A manufacturer is not sitting in his office waiting for your idea to show up on his desk. He is busy running his company, dealing with employees, wholesalers, retailers, shippers, advertisers, quality control, etc. He is also interested in the company growing. He can do that by increasing the number of people buying his current line of products, improving on a product or introducing a new product.
Many larger companies have research and development departments that do just that.
However, other large companies and smaller ones will entertain ideas submitted by private inventors. Imagine you have a small busy Widget company. You are in your office reading your mail. In amongst stacks of bills and orders are several proposals for new ideas. You would be interested in a new product that will help you expand your market if you could find the right one.