NUMA-goers are very impressed with tilesets that look fancy or professional. In fact, one important part that people look for in a map is the tileset. With a better tileset, it will be more likely to attract someone's attention when they look at the thumbnail image on NUMA.
See more about tilesets in this article: mapmaking.
This category was recently implemented onto NUMA. Its purpose was to allow maps that users called Incomplete and were actually just custom-made tiles for others to experiment their mapmaking abilities on to have a Map category of their own.