Remember the Lite Brite? Okay, say you have a nice design that's 10x10. Someone else tries to create something similar with full size light bulbs, in the same size, so it's three bulbs by three bulbs. They're the same size, but you can have more detail with the lite brite because it's using smaller and more individual points of light. While the bigger light bulb can in theory be set to match the "average" color from the smaller bulbs (i.e., if there are two red, two blue, a yellow and a white, it may be a softish purple shade), it'll never be able to replicate the specific color set.
If you were going to out put them on a video stream that was 10 pixels by 10 pixels, the litebrite version would basically set each pixel to the appropriate color and be done, while the light bulb version would have to guess at the specific colors for each pixel.
Although that sounds like a simplistic example, a lot of the display boards at large sports stadiums used the same concept. I know for a while I ran the one in Jacksonville that was used for Jaguars games, Florida-Georgia, the Gator Bowl and other college games until they renovated the stadium for the Super Bowl. Basically it had each "pixel" as a square of four light bulbs, red, blue, green and white, and you could approximate colors by setting specific combinations on.
