No offense but why are you so condescending? I agree the OP should have done some research but why not just tell him without the sarcastic tone and dots...
Because this question is asked time and time again. No new things have come up in phone protection since the iPhone 6 Plus launched that would give different answers today versus the answers that were given in September 2014.
Even going back over 20 years to a time when phones first came out of the bags that held them and their batteries, there haven't been significant changes to the case/no case argument. Screen protectors and phone skins only go so far in protecting a phone.
Pros for using a case:
1. It protects the phone in the event of a fall
2. It adds to the ability to grip or hold the phone
3. It protects the phone from nicks and scratches in day-to-day use and storage
4. It makes a fashion statement
5. It helps to distinguish your phone from others if several phones are placed on a table.
Cons for using a case:
1. It adds bulk to the phone
2. It hides the look and beauty of the phone
3. The case may get in the way of the speakers/mic/camera if it has a loose fit
4. The additional cost.
I'm sure that I left some out. However, these answers haven't changed since the Motorola StarTac was the most popular phone to have.