Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water.
The majority of photosynthesis happens in those fan leaves. Removing them would reduce the amount of energy the plant can produce. Cannabis is basically an annual plant meaning it lives, flowers and dies. The plant spends it life feeding itself and storing energy to produce flowers and if allowed seeds which is the end of the plants life cycle. Reducing the amount of photosynthesis taking place by removing the fan leaves reduces the amount of energy the plant can make, store and use to produce flowers.