It greatly improves your comfort level, if you need to be without a regulator for a bit. For example, if you lose your reg and have to do the sweep to find it, you are going to be far less anxious if you know you can easily do without breathing for a minute or more (because it certainly won't take you that long to find it).
I was watching students in the pool last night, and when they take a reg out of their mouths, they're almost frantic to put one back in right away. It's a nice thing for a diver to be much more relaxed about it than that.