A simple way to put is that soil acts as a decent buffer for ph imbalances nutrient strength problems and helps with micro nutes with hydro you and your water are responsible for everything the plants gets except light and ventilation so if you ph goes ways off or nutes are off etc. etc. the consequences are immediate and can go bad real quick, like loose everything within a week quick with some noob grower mistakes. Because you controll EVERYTHING with hydro it can be real picky if you don know what your doing.