In what cases should we separate the faces when exporting?

A while ago, I tried exporting an object to another 3D app, just for testing.
And then I became surprised seeing some edges disappear. I thought well, maybe it’s the other app’s bug. No worries, I could re-add those edges manually.

Then I realized that the faces were actually separated. I was like, “what” ? How??

And then I looked back again inside Nomad Sculpt export menu, and found that it automatically checked the option to separate faces.

I then unchecked it and re-imported the object to another 3D app everything was good, there were no disappearing edges, it’s all good.

Now my question is, in what situations would we need to separate the faces while exporting?