Note: This page is taken from n.wikia.com.
Purpose of the tileset category is 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.
A tileset is the combination of tiles in a level or map. Some maps are created with only the tileset so that others can experiment with objects without working on a tileset.
N users 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.