The correct answer is - because it increases production of glucose, which is the plants' food.
The main reason why the plants started to move from the water towards the land was that on the land they were able to produce more glucose, and the glucose is the food that the plants produce for themselves.
The increase of glucose production on the land was possible because the plants received more sunlight. The sunlight is crucial for the process of photosynthesis, in fact this process is not possible if there isn't sunlight. By using the sunlight, as well as gases from the atmosphere, the plants manage to create their own food, glucose, and the more they produce of it, the better for them, as they can grow more, live longer, and be in better condition.
