If you’re a hard gainer - meaning you have a hard time gaining muscle & size - you might want to try a weight gainer. Many hardgainers are suited for dirty bulks which fast-track gains by providing indiscriminate calories regardless of the source. For other body types that are more prone to fat gain you might want to keep those calories cleaner and more precise. This is called “clean bulking” and clean gainers are built to provide this more precise, high calorie macro nutrition to athletes.