Often in the room, the ambient light mixes from the light in the room that has a different color and mixed by the sun from outside that comes into the room through the window. In addition, the light in the room is usually very little, so although it has been using a lens with a very large opening, it still has to raise the ISO high enough, resulting in decreased picture quality due to high noise and color saturation is lost.
From here it is known that relying on the lens and environmental light alone is often difficult to get the results of the image in accordance with the desired. Therefore, buy Nikon flash is a good investment for photographers.
Therefore, how is the conclusion? Buy a lens or an external flash? It’s all depends on yourself. If you like outdoor photos naturally, such as landscape, street photography then most likely flash is not an important accessory. But the accessories such as sturdy tripods, or quality filters may be more important. But if you like working in the studio so that it can work anytime day, night, dawn, and love the ability to control the total lighting, then investing into the flash system is more important than the lens.