I wouldn't suggest that they help prevent cracking - only good padding around the edges will help much there.
But they
do help keep shattered glass in place. If a screen without a protector breaks, the first thing you want to do is put one on.
The 4 and 4S has
no protection around the edges of the glass at all. This likely accounts for why I've replaced more screens on those than any other device.
Edit:
This article makes for interesting reading. It mentions that Gorilla Glass is created by creating a pressurised seal around regular glass, which helps prevent it from scratching and from cracking.
Should that seal become scratched, the pressure is lessoned at that point - the glass becomes more likely to crack there.
For such displays, a screen protector could indeed help prevent cracks!
It's rumoured that the iPhone 6 uses a form of Gorilla Glass, but I'm not certain that anyone knows for sure - Apple seems to be keeping it under wraps.