Sky replacement has been fantastic and worth the price of Luminar alone.
The reason? Masking.
Masking a sky in Photoshop can be a pain. It's easy enough when you're dealing with buildings and well-defined mountains, but much harder when dealing with foliage, long grasses and other things with elements of transparency.
The trick here is to use sky replacement to separate the sky, rather like a greenscreen, then, with a few adjustments, use that as a mask in Photoshop. Then you can apply curves, colour corrections and other normal adjustments to the sky, without affecting the rest of the scene.
All you need to do is create a 'sky' for the program to use, consisting of a single, flat colour that doesn't appear anywhere else in the scene (or, at least, anywhere else close to the sky). Use that as the 'sky' for the purpose of replacement, then export the result back to Photoshop and select for the colour you replaced the sky with.