Why do mirrors have both glass and metal? The metal is the reflective part - what purpose does the glass have?
Possibly as a shield to keep it from getting scratched or rubbed off.
Two reasons:
#1 to hold up the relfective part (commonly the silvering bit on the back of the mirror)
#2 To protect the reflective part
Glass is the common item of use since it is easy to work with and is transparent.
I’d just like to mention the only exception to this that I have seen. Some friends had a mirror where the reflective medium was a thin film of aluminium on a thin plastic sheet. Probably mylar. This was stretched over a rigid aluminium frame that must have been about 2x3meters. The whole contraption cannot have weighed more than 5 kilo!
The surface was no longer pristine, but the overall effect was impressive.
The reflective part is a peice of metal. It needs to be ‘held’? I didn’t know it was that sensitive 
Again, it’s a peice of metal. Glass protecting metal seems drawkcab-ssa.
I thought it might be that covering the metal with glass means you don’t have to polish the metal but you have to clean the glass (or so I’m told) so does it really save you any effort?
Lazarus: It’s usually very thin metal foil. It needs support and protection.
Is the metal more expensive than glass? If so, that would explain why we substitute glass for metal in mirrors…
No, it’s not simply because metal is any more expensive than glass on a unit cost basis it’s mainly that the process of manfacturing a very accurate reflective surface that won’t corrode is more easily and inexpensively done with a thin metal foil surface deposited or otherwise bonded to a flat glass surface thereby combining the benefits of both materials and retarding corrosion once the metal-glass bond is sealed. This is more easily done than trying to make an equally accurate metal mirror which would be more expensive, corrode fairly quickly and be more subject to scratching (if portable) than a glass surfaced mirror.
Question asked and answered.
Thanks for all the help.