Most startups begin with an entrepreneurial product idea by one or more of the founders. The founders then develop the product, hiring a product manager only after the product is ready for release.
If you're one of these founders, you don't want someone coming in and polluting your idea. You probably also have some experience dealing with the market you're targeting. But while industry experience and an idea can be a great start, your product will probably fail if you don't do some product management before and as you develop it.
Don't decide the positioning and marketing strategy for your product after development. The most successful products are those whose development reflects a well-conceived positioning and strategy. Product management is therefore important long before you're ready to release - or even begin marketing - the product.