D&D General Weapons should break left and right

[Lanefan]

Why should the GM call that out before the players ask about it?

If it's something the PCs know they're getting into e.g. they're intentionally travelling across a desert known to be there, calling it out makes sense.

If it's something the PCs might not realize they're in until it's too late e.g. they've just been teleported to a barren Atacama-like area that has no water within a hundred miles, then it shouldn't be called out up front.