Reverse engineering is useful when you are looking to copy the functionality of an existing. The legality of reverse engineering varies from jurisdiction to jurisdiction across the world. So please check your local laws before taking apart a product.
Conventional engineering is useful when you are designing a product that goes into a new area.
Very often reverse engineering is costly and complex. It is often simpler and cheaper to get the functionality by engineering the product using conventional means.
In most cases where reverse engineering is used to develop a product, it is combined with conventional engineering. Results used from reverse engineering are used as a guide in developing the product.
Most companies buy and use competing products to learn their competing features, even though they may not take them apart.