why should we destarch a leaf before conducting an experiment to show that light is necessary for photosynthesis?

In the above mentioned experiment we show that in presence of light leaves of one plant is producing starch by the process of photosynthesis and the another plant leaves which is kept in dark room is not producing starch due to absence of light. The leaves of both the plants that are used for experiment already contain starch. To perform the experiment accurately it is necessary to remove the pre existing starch from the leaves. Otherwise we cannot be able to prove that photosynthesis is occurring only in leaves exposed to light, not in other one kept in darkness. If we do not destarch the leaves in the beginning, both the leaves, in light and in darkness will show the presence of starch. The experiment will not be successful then.