There are many myths and legends promulgated by sunglasses manufacturers, particularly of "name" brands.
Let's consider some facts:
Lenses are made either from glass or polymer material. While glass is hard, polymer can be too. The wear resistance and optical correctness of a lens has little to do with the material from which it is made. Glass should not be worn if there is a risk that it could shatter, but glass can be toughened to reduce this likelihood.
Tints remove things from light -- they cannot add what is not there to start with.
The thing people generally want to remove from normal sunlight is ultraviolet. $20 sunglasses can do this just as well as $200 sunglasses.
Polarising lenses are particularly effective against removing glare, as are umbral lenses. Both come in a range of tints, depending on your preference. Both do their job better than bog-standard UV reducing sunglasses.
The colour of the tint affects the colour of what you observe (surprise, surprise), so choose your tint based on what you prefer to see, not on how you prefer to be seen!
Photochromatic lenses (ones that go darker in UV light) work well when they are directly exposed to UV light. This means they don't make good driving glasses (especially where drivers sit out of the sun). Their effect also diminishes over time -- they increasingly go less dark and less light, eventually stabilising at a mid tint.
You pay a premium for a "name".
A lot of fashion sunglasses (particularly those of the pale pink, pale blue or just plain pale) impart no benefit to the wearer whatsoever, apart from coolness.
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