As well as providing biofilm and natural food, dried leaves also contain tannins and humic acids which leach into the water having beneficial properties including anti-bacterial and anti-fungal. They can help promote good health and color, stimulate breeding, reduce stress and aid in the molting process. Tannins are what give blackwater rivers their characteristic tea-color appearance and low pH value. Although the leaves will lower pH of your tank, you would need to use more than the standard dosage quantities recommend for shrimp tanks to have any real effect.
There are benefits to using more than one type of dried leaf in your tank at the same time. Some leaves provide different beneficial properties than others, they break down at different rates and so there will be a constant source of natural food in your tank and you are also providing a varied diet for your shrimp.