Let's talk about light coming from a light source and being seen by observers - us. The light source generates a given amount of energy every second. We see the light when it enters our eyes and falls upon specially sensitive nerve cells at the back of our eyes. The brighter the light, the more energy is entering our eyes every second. So, a light source gives off energy in all directions, and, as that energy moves away from the light source, it spreads out and therefore becomes dimmer. To see a light at a great distance, it must be sufficiently bright that, after it has spread out across that large distance, it is still bright enough for us to see.
Of course, one way to control brightness is to use magnifying techniques. If we could gather up all the waste light travelling off in other directions that would not have reached our eyes and focus it into a beam that does reach our eyes, then we could make it much brighter. For the moment we will ignore this and return to it for a full discussion later.