The extra rigid sole unit prevents the boots from flexing excessively across the toes, the common point for other boots to fail.
This is what a website says about a certain type of boot. My personal experience is the inverse. A boot which has a more flexible sole lets you feel a small stick before it breaks, lets you feel rocks when you climb them and most of all, more flex seems to keep your toes warmer. My impression is that toes get cold quicker in stiff boots.