By turning on that option, you are disabling Static/Stationary lighting (Lightmass), and switching over to completely Dynamic lighting. This has a higher cost performance wise, but can save time while building a level since you no longer need to build the lighting. For large open worlds Dynamic lighting works great, but the quality of lighting & GI is not quite as good as pre-baked lighting.
There is nothing wrong with using Dynamic lighting, I do all the time, I just wanted to let you know what you are disabling. 