Usually it's used for lightmaps (example on Unity, or from polycount wiki, also on Unreal [1] [2]) . Talking about Unity, note that I'm not sure about that, maybe the detail maps are using UV2? (if a Unity user can confirm)
Take a look at microsplat on the unity store (it's free) to get an idea of some other things you can utilize second UV channels for. Macro/micro textures, height based blending, etc.
In addition to lightmaps and detail maps, you can basically make any part of the model have exactly the UVs that you want a specific texture to have. It's useful when blending between different textures/material sets with whatever masks or vertex colors. Least in Unity HDRP's built-in Lit shader you can make use of the…