Not everybody works the same way, and everyone has different needs. You will find some people who can maintain healthy body weight or gain muscle or lose fat without using any supplements, but for some people, it's simply not as easy. So, in that case, people should go for mass gainers. For example, let's say you need 3500 calories to be in 500 calorie surplus, and if you can eat that many calories with a macro balance profile, there is no need to even think about these supplements.