The catalyst is defined as a chemical substance that helps increase the rate of the chemical reaction without being changed or affected by the reaction itself (the catalyst is not involved in the reaction, it only aids with increasing its speed).
Based on the above definition, adding a catalyst to the reaction:
H2 + I2 .........> 2HI
will increase the rate of the reaction. This means that the reaction will proceed faster and the product (HI) will be formed faster that the case where the catalyst is not added.
Hope this helps :)