It’s like Vertex Groups but with faces. Sometimes you want to save a groups of faces like this. You can’t do it with Vertex Groups: if you press “Assign” with these 3 faces selected, unselect and then press “Select”, you end up with 4 faces selected. Not anymore with Face Maps.
I also see another uses somewhere (like a new, nice way of rigging without bones… but I’m also not sure because there is almost no documentation).
Interesting. Is there any way to use face maps to influence modifiers or something instead of vertex groups?
Based on the description, it sounds cool. Rigging without bones, but I haven’t the slightest clue how to use it.
I just found more info/links in this thread but… I see now that this feature has some issues. In this discussion developers say no one is currently working on this. So even with this early implementation already in Blender (the Face Maps panel) this feature could be removed soon 
https://www.bilibili.com/video/BV1gX4y137BM In this video, from9:40: to 13:17, he used Face Maps to create a pattern
Face Maps have been replaced by a more powerful Unified General Attributes system. Here’s how you can assign Face Maps using this new system: https://blender.stackexchange.com/q/337742/142292
