The phloem is essential for transporting sugars the plant has made to other areas of the plant, this is necessary as some areas of the plant cannot perform photosynthesis or perform it at a much slower rate. E.g. the roots can't perform photosynthesis to get the sugars it needs because they aren't exposed to light, but they still need energy to function and without sugar transport by the phloem they would not get this so areas of the plant would die
