I wish I could show you the local broadcast of women in niqab picking fruit. My Orientalist brain inferred that they were “shy” (maybe piously so) to show their faces to the TV world. My colonized Moroccan tour guide’s brain told her that they were protecting themselves from being blackened under the sun.
Many reasons and many ways to wear niqab. Don’t bother to guess. And asking might not get you the truth anyway.