Grow tents are reflective on the inside because they have a reflective layer (usually mylar - about 95-99% reflective) lined on the inside. This will help ensure that the light that comes out of the grow light can be used to its maximum possible efficiency and that the plant can benefit out of it. This layer ensures that all the light bounces back towards the plant and from all the different angles, ensuring the plant gets optimal lighting, therefore a bigger and a better growth and higher yields.