[Reach]

Think about it this way.

The bacteria that form cavities favor low pH environments and require sugars actively in your mouth to do their business.

So if you have something for breakfast with high acidity content, and don't brush your teeth, it's going to sit there all day. This is the case for most people, since they like to eat cereal. Your saliva will help to bring the pH back up in your mouth, but most people eat throughout the day and will end up keeping that pH pretty low and that can be problematic.

This gives bacteria plenty of time to set up plaque camps, at which point they can start producing lactic acid all over your shit.

So, brushing your teeth after you eat will help to quickly restore pH balance and deny bacteria the substrate necessary for plaque formation until you each lunch. That gives them a much smaller window of action, in which case you shut them down again before you go to bed.

And of course, combine with flossing once a day after your last meal and you'll probably never get cavities. Voila.