Sunglasses perhaps not the most important accessory but indispensible in some special occasions. Summer or beach looks for example. Though they are strongly recommended for certain outfits.
A modern casual look means wearing free clothing and many accessories. And sunglasses are one of them. Classy look may be supplemented with transparent sunglasses with a slight tint. Even sportswear looks great with sunglasses. Of course they must be suitable. Thank to the fashion brands we have an incredible variety of forms, colors and sizes. Strict or joyful, classy or aviators, the assortment is simply impressive.
In general you can use them in any occasion with any look.