1. You stop losing your earphones. I say you stop, because I know that majority of us lose it. It just happens. Earphones have become such an integral part of us that we literally carry it around everywhere. So, it's just easier to keep it safe when you have a case.
2. No more tangles. I know some people pride themselves over their tangled earphones. It just comes out like: oh god, my earphone is always tangled up, you know! Total feign. But, those who want it to be straight and clean (and usable), then use a case. It actually works. And, earphones that are not tangled up all the time actually lasts much better.
3. You can carry it easily. You want to keep them in your pockets, throw them into your bag, carry it in your hand- the case makes everything easier. It may look like a event in itself to take it out and put it back in the case, but that's really not how it feels once you get used to it. I am so used to putting my earphones back in it now, that it's almost impossible for me to keep my earphones without the case!
Finally a brownie point- the cases are really cute. I love mine! I have a case with a metallic top, so it kinda sticks to the magnetic part of my laptop, so even the case doesn't get lost!
My dad's first comment when he looked at my case was why I didn't get more of them for all of them in the house! It's cute, it's protects your earphones, and saves you a lot of money.
If you don't use it, take it from me that you should definitely think about buying one the next time you spot it!