An organism that produces catalase enzyme is protected from the harmful effects of

Protected from the harmful effects of free radicals, often referred to as reactive oxygen species (ROS).
Hydrogen Peroxide (H2O2) is a by-product of metabolic reactions of cells that use oxygen (aerobic respiration) to produce energy from carbohydrates (food). The problem with H2O2 is that it can be toxic to cells, due to its free radical properties, as an ROS. Organisms protect themselves from this by using an enzyme called peroxidase, also known as cayalase.