If you are already feeding a complete pet food, there is no need to supplement additional vitamins and minerals. Almost all pet foods contain the appropriate amounts of nutrients already. Supplying additional nutrients such as minerals can even harm your pet. For example, a mature cat only needs 0.6% calcium and most cat food contain in excess of 1%. Why would you even think of adding more. Older cats often suffer from high blood calcium levels and less calcium is recommended not more. Another example is vitamin D3, everybody knows that people need more so we think our cats are the same. In fact, mature cats should receive less D3. Excess D3 may cause excess calcium to be absorbed and can lead to high blood calcium levels. Vitamin infused treats should always be viewed with caution.
If you are considering a supplement, call the company that manufactures the pet food you are feeding. Ask them if a supplement with their food is a good idea. There are few instances where it's beneficial to use any supplement when feeding a complete pet food.