What’s the point/advantage of a steel mirror?

5e D&D lists a number of mostly mundane items in the PHB on page 150. One of them is a “Mirror, steel” (5gp, 1/2lb). Initially I thought it to be a steel framed mirror, but I think this is actually meant to be a polished (to the point of a reflection) piece of steel.

My question is two-fold:

  • Is this a steel framed mirror or a single piece of polished steel?
  • What’s the idea behind a steel mirror, instead of a glass mirror?

Is it that glass is more expensive and/or too fragile? But 5gp for just a piece of steel feels (relatively) expensive as well. I am aware of the possible idea that it might reflect vampires, but I feel like that may not be the RAI.