I’ll try and describe a really simple example here,
I have a lovely detailed wall piece that I’ve created and want to use multiple times in the level. This piece shouldn’t have it’s own light map because it’s being repeated? For example in another area there may be a statue casting a shadow on one of them. So then how do people lightmap modular levels within Unreal? All I can think of is to create the full level in a 3D application and divide it up into chunks with their own lightmaps.