Benefits
Baking lightmaps to diffuse is a very cumbersome process that also bloats the project up, having separate UV for lightmap allows artists to create beautiful scenes that are performant. In addition, having lightmaps this way enables artists to utilize the workflows they are already using.
Update: Unreal Engine
https://forums.unrealengine.com/t/light-bake-export-option-for-gltf-exporter-5-2-preview-2/885116
image
Test Space
Download: https://booth.pm/en/items/1943320