Let's suppose we can only see light that has an intensity of 1 watt per square metre. How far away can we stand and still see the light? Well, if we call the distance d then we write the equation 400/(d x d)=1 from which we can easily see that d evaulates to 20 metres. So, at a distance of 20 metres, we can only just see the light. If we move any further away, we can't see it. So, if we need to see the light at a greater distance than 20 metres then we must make it brighter and hence we must make it give off more energy.
Obviously, we have used some simple numbers here just to explain the principles. You might like to consider that, during the day, everything we see is due to energy released by the sun, an immensely bright light source. That energy left the sun at a distance of 93 million miles (150 million Km) from Earth and was dissipated according to the inverse square of the distance between the sun and the Earth, yet it is still astonishingly bright, having taken 500 seconds to reach us. At night, we see stars that are an incredulous distance away. The light has taken many millions of years to reach us - yet we can still see it. How bright must those sources be?