Yep! You can think of a cubemap as virtually surrounding whatever it's applied to, so whatever has them applied is inside (reflectively-speaking) that cube. Unlike regular, single-image environment maps (like tcn01__ref01.dds or ttr01__ref01.dds) which are basically projected from the your camera's view onto whatever is mapped with them, cube maps are fixed and do not move, making cube mapped objects appear to more realistically be reflecting an environment they're in.
As far as modding possibilities go, cube maps are (off the top of my head) primarily used for two things: First, they're what's reflected off of bumpy-shiny things, like shinywater. Second, because they represent the reflection of an environment, they're also used in .set files, I believe. So, one could make/generate a mountainous or desert-y or swampy cube map that would be reflected off armor, for instance. Though they can be very detailed and complex, they can also be blurred to give more of a "feel" to what a tileset, for instance, has going on, light-wise. IIRC, you can always apply a cube map to something that would have worked with a regular environment map. However, some things might look better with one than the other, depending.
Again, environment maps are single-image things which move as your camera does. It's why the eyeballs follow the camera in this video. Cubemaps are fixed, and you notice the reflection change when you move the camera, like in this one.
Edit: It's also worth pointing out that ttr01__env is actually a pretty terrible example of a cube map. In most normal cube map situations, ttr01__env5.dds/tga would not reflect clouds but he ground/surface.