The Apple iPhone 4 bumper has 2 jobs.
It was originally released to help the iPhone 4 cope with it's singal problems, but it also protects the iphone for damage.

I have numerously dropped my iPhone by accedent and luckly it's always landed on its side. This case protects the iphone from any major damage it would have suffered otherwise. It absorbs impact and protects the corners from scrached or dents.

My iPhone 4 didn't have noticable signal problems, and when it did I would put it down to my terrible network provider. However, the science is behind it. This case bridges the gap between the 2 aluminium plates at the bottom left of the phone which are causing the infamous signal probelms. By bridging this gap, Apple have manage to fix the signal problem without a major redraw of the iPhone.

Best of all this product is free. Courteous of Apple.