It’s not likely a mesh won’t need a second UV channel, sometimes there are cases where your UV’s already follow the guidelines for lightmaps, in which case you don’t need to.
The reason it doesn’t tell you when you might need lightmap UV’s is because you might want to use the first UV channel for your lightmaps in which case you don’t have a second channel, or you don’t intend on using lightmaps. Even in some cases even if it gives you an error about your lightmaps they might still turn out to be fine–that’s cases where for instance I set them up what I think is correct but there might be a small error somewhere that it’s detecting but it still renders fine.
Generating lightmap UV’s in UE4 doesn’t usually give a good result, it’s just an option if you don’t care about spending the time to do them properly.