When I lived in New York, I’d occasionally see photography crews shooting store windows in SoHo. To eliminate reflections and window glare, they’d erect a gigantic black fabric screen behind the photographer. It looked like a real pain in the ass.
Another situation in which reflections are undesirable is when shooting through the window of an observation deck. Photographer Josh Smith, frustrated with reflection-marred shots he’d taken on a trip to Japan in 2015, thus invented this Ultimate Lens Hood:
Here are examples of how the ULH makes a difference:
It can also, if flipped back over the camera, offer some protection to the camera body from rain: