Why are transition metals used as catalysts?
You need to remember that a catalyst is a substance that speeds up the chemical reaction changing the original pathway of reaction.
You should remember that 10 is the maximum number of electrons that d orbital can hold and the transition metals have this d orbital not filled up with all 10 electrons and precisely this d orbital feature makes transition metals to give or keep electrons, at different times, while chemical reaction happens.
You need to select the transition metal depending on type of reaction such that: the hydrogenation reaction needs to be H2 chemisorb, hence transition metals such Ni, Pt, Co represent good selections.
You should also know that the oxidation-reduction reactions may be catalyzed by transition metals.