Why store your sunglasses in a case?
Sunglasses are fragile and there are times when you don't wear them on your nose.
The glasses case is useful for storing them at the end of the day or when the light level drops, without any risk of damaging them.
When you are not wearing your glasses, they need to be protected so that the lenses don't get scratched and the frame doesn't get knocked or deformed.
Often, it is when you remove them that your sunglasses are at the highest risk of getting damaged. This is why the case is an essential item for storing and transporting them.