Well, I am not sure I can be more explicit than in the wiki page. But here's another try to explain this to you in simple words:
1) applications install udev rules
2) these are executed at early boot when the hw is detected
3) these rules fail, since /usr is not around
4) later on the apps won't see the devices properly, or can't see some of their features since the rules didn't get executed, and the udev devices lack the properties that should normally have been set had the rules been executed properly.