These provide very nice looking visual effects related to the environment around the object. For instance; shiny armour can reflect the sky and the ground, rather than just being "slightly watery looking metallic".
Sorens Information on the topic
This is a rough copy from the Discord about environment maps and cubemaps:
Regarding environment maps: what wasn't working before was cubemap environment maps (except for water). It's not something that's been used by that many this far (I don't think I've even seen any releases) but it gives a far superior reflection imo
This is an example shot from something I was dabbling with (actually 1.69)
What you should pay attention to is that with cubemaps, the reflections better reflect the environment and the position relative to it (contrary to the spherical environment mapping that sticks with the camera angle)
So like in that screen, you can tell that, sand is being reflected from beneath, sky from above
This is an EE shot, obviously just testing functionality
Of course, to benefit from it, we need cubemaps to be made for tilesets, but there's ways to make that easy
Regarding how environment mapping in general works now:
It isn't something you have to set up a model to use explicitly, but rather it's modulated by roughness and specularity (which is the correct physical behavior). That means that you can just supply a spec+roughness map and leave the envmap 2da entry to ****
This also means that you can then use the alpha channel on the main texture for actual transparency, while still getting environment reflections
The new is that if you supply a roughness and/or specularity map, you'll start to see environment reflections too, even if no envmap is explicitly defined
Cubemap Generation
The way I did is was by making a generic area, take screenshots in all 5 directions and some hand-made sky/ceiling
This is for the city exterior:
At that point, I embedded the some lightsources for reflection, but shouldn't be there anymore with actual specular lighting
You can try yourself if you put this in your override:
But you'll need 8193.14 build for now