If your map is too large (in px used on the VTT), the game can exhibit decreases in performance. So, lets say that you wanted to have a single battle map for something taking place over 2000 squares x 2000 squares (140,000 px X 140,000 px). This is exponentially larger than the largest recommended map size, and twice the height and width as the largest map I've personally ever successfully loaded. The limit here is not the number of squares (although it does have some impact), but the sheer size of the map. If however, we reduce the size of the cell, we can use a map with an area a fourth the size (70,000 px X 70,000 px). This is still a massive map, but is much more manageable for systems to load than the full sized map. And of course, we can go lower to make it even more manageable. Note: The sizes shown above are way past what I would ever recommend running with purely to demonstrate the principle. EDIT: And an alternative use. Not everyone has the skills (I sure as heck don't) or time to create their own custom battle maps. If you wanted to use a battle map, but couldn't get it at the proper pixel resolution, you could change the square size to use it at the lower resolution, but still get the same play space out of it.