Given the strength and scratch resistance of Gorilla Glass, are cases of value for iPhones? Do I need a protective case for my iPhone 5?

This is my first smart phone. I thought one of the advantages of a modern smart phone was the Gorilla Glass and the small size and weight. It seems to me that cases add bulk and weight to the phone, so I find it odd that cases are so common.

Any people out there with an iPhone without a protective case? What has your experience been. It is rare to see a raw iPhone on the streets. Sometimes I think this is just a) security theater (i.e. makes you '''feel''' like the phone is more safe) and b) a way to personalize the now ubiquitous device. 

There's an article/video that talks about the efficacy of Gorilla Glass. The summary is it is quite good but that small particulate matter (sand) can scratch the glass. Typical things like coins &amp; keys in the pockets should not scratch the screen.

Given that I feel the glass needs no protection, what value does a case offer?
I have an iPhone 4S.  Like you, I originally thought that I ought to get a protective case for my iPhone.  I tried several cases, but none of them were satisfactory (too bulky, too cumbersome to open to get to the phone, too ugly).

I now have no case for the phone, and I keep it in my trouser pocket all the time.  Instead of getting a case for the phone, though, I got a key case for my keys (which I keep in the same pocket) to prevent them scratching the glass!  That solution has worked well - the iPhone is now over a year old but looks as good as new, and I haven't added any size or weight to it.