Why do transition metals show catalytic properties?

The most important reason transition metals are good catalysts is that they can lend electrons or withdraw electrons from the reagent, depending on the nature of the reaction. The ability of transition metals to be in a variety of oxidation states, the ability to interchange between the oxidation states and the ability to form complexes with the reagents and be a good source for electrons make transition metals good catalysts

Transition metals and their compounds are extensively used as catalysts due to the following reasons:
(i) Their partially empty d-orbitals provide surface area for the reactant molecules.
(ii) They combine with reactant molecules to form transition states and lowers their activation energy.
(iii)    They show multiple oxidation states and by giving electrons to reactants they form complexes and lowers their energies.