In a typical mirror there is a thick layer of glass followed by a thin layer of silver coated on one side of the mirror which reflects most of the light, what purpose do the glass have?
Silver sheet is expensive, and it deforms easily.
"Float glass" is quite smooth and flat, and smoothness & flatness are crucial for a good mirror. It is also very stiff and highly elastic, so leaning on it will not usually damage it. Simply painting the back of a sheet of glass makes a usable mirror: reflection off the front surface amounts to a few percent. Not nearly as bright as silver, but cheap and optically good.
It turns out that it's easy to deposit an extremely thin layer of silver on glass. The silver is very highly reflective, and conforms to the shape of the glass surface; so silver-coated glass makes an excellent mirror.
Unfortunately silver oxidizes and stains easily, so a silver-coated mirror could become junk in a few days. The solution is to coat the silver onto the back of the glass, then cover the silver with a protective layer of paint.
The few-percent reflectivity of the front surface of the glass is sometimes a problem, because it produces an image that is at a slightly different distance than the primary image reflected from the silvered back surface. The primary image is usually so bright that you won't notice the front surface image, but it's not very difficult to prove the front surface image is there. For example, try reflecting a laser pointer at an angle off of a bathroom mirror, and look at the reflected spots on the wall.
The glass is there to protect the “silvered” (aluminium for a lot of mirrors) reflecting surface from corrosion and damage whilst being handled.
Such a mirror is called a back-silvered mirror and there will also be a protective coating of paint/lacquer on the back of the reflecting surface.
For mirrors of optical quality (manufactured to tolerances of a fraction of the wavelength of light) are usually front-silvered and so have to be handled with great care.
The potential distortion and absorption of light by a layer of glass is thus avoided in such mirrors.
First The glass substrate cutted and polished with special radius of curvature, then a thin layer of Al,Au or Ag is deposited on this substrate and form a mirror with specific focal length. So we can not have curved mirror only with thin metal layer, you need the substrate in deposition process in chamber.
Second, optical characteristics depend on radius of curvature. And for a mirror with certain focal length you need a substrate that is robust and tight under pressure and stress and temperature changes. Because if these parameters channged, caused the materials change their dimensions and this leads to change radius of curvature.
Most glasses have very small changes with stress, pressures and temperature changes, so we use them as optical substrates. (Specially BK7)