Your phone is too thin and light
The trend of phone market is making a phone as thin & lightweight as possible. On the other hand, phones would not be durable like before. For peace of mind, we would buy a case for extra protection. Certainly you don't have to use a case if you are using a Nokia Windows Phone. It's built like a tank.
Your phone don’t have insurance
True, some people do spend an extra $10/month insurance for their phones in case of dropping and losing. But the rest of us, in a majority, would prefer to take care of our phones, such as dressing up your phone with a case. The case is your phone insurance.
You take your phone everywhere
When I get a new phone, it always go full naked at first days, because I think my device is awesome and I would wish to keep using it that way. I take it everywhere and soon I would found my phone is full of scratches and scuffs which would be caused by keys in my bag or hard, gritty sand in my pocket, and it make me feel brokenhearted. Later I would use a case, a nice slim one that adds a scratch/drop protection, but still keeps the overall style of the phone.
Your phone would be safe in a case
If you throw your phone off a cliff or throw it across a room, cases aren’t offering great protection. But if your phone accidently drops from desks, counters, slipped from your hands, or been knocked to the floor by cats, cases would give your phone solid protection.
Your phone is expensive
Spending $30 on a phone case seems to be a little bit expensive, but if you compared it with the cost of iPhone $650, you would be feel at ease to spend $30 to give your precious iPhone an extra protection.
If your phone is an older model
New phones and new models come out every year, even every quarter, but I still love my iPhone 4 though the iPhone7 is going to be released. However, using an older model would make me feel like an old-fashioned girl, which is embarrassed especially when hanging out with my friends and many of them have a new iPhone. At this time, phone case could be a good way to decorate or hide the fact that you are still rocking an older phone.