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Keeping your glasses in a case can help keep your lenses from getting scratched or broken.
This is a personal preference. Most glasses will come with a specialized case for that brand. A soft case can be used for multiple things and if you have more than one pair of glasses 2 hard cases can be bulky to carry in a purse or backpack.
That depends, glasses and phones come in all different sizes. If you have a larger pair of sunglasses and a thin pair of readers there is probably enough room to carry a phone as well.