yeah but isn't there already a gigantic light in the sky rotating for free ever day?
Yes, but it is below the horizon when it's in the north and most rooms don't have 360 degree windows.
That's the thing - when the plant is outside it will get sun from multiple directions as the sun moves, but when it inside the light will only be coming from the window.
And even outside the sun is shining more from one direction (south here in the northern hemisphere), since it is below the horizon at times, which will often cause plants and trees to have more branches and leaves facing that direction.