Why is it necessary for certain chemical reactions to be catalyzed

Because certain reactions are so slow that the reactants do not react at an appreciable rate and products are not formed.

It is necessary for certain chemical reactions to be catalyzed because without catalyst the pathway of the reaction would have such activation energy that the reaction would not proceed or the rate of the reaction would be so slow that in practical terms the reactants would not react.

Explanation:

Catalysts modifiy the path for the chemical reactions lowering the activation energy.

The activation energy is the energy that the reactants must acquire in order to the collisions between the substances have the needed energy to break the bonds and form new products. In this form the rate of the reaction is increased such that the formation of the products is relevant.

Catalysts do not modify the energy of the reactants but provide a new pathway for which the activation energy is lower making that more collisions effectively yield the new products.