Desorption is process in which a substance is released from or through the surface. A catalyst proceeds in helps in increasing the rate of reaction rather they involve in the reaction. Consider, catylytic action, the reactants gets adsorbed on the surface of the catalyst & gets converted into the product. The product or the unreacted reactant has to be removed from the surface of catalyst. At this point the desorption helps in removal of the reactant & product form the surface of catalyst & makes its free for the next reaction.