U need to brush mainly before. Because when u sleep ur mouth produces less saliva. So that makes the bacteria in ur mouth double faster than any time of the day. If u don't brush then ur basically eating with a lot of bacteria in ur mouth.
If you brush thoroughly at bedtime,which you should, then eat breakfast first . There is no real benefit to brush before. Brush both before and after if you want to but maybe just a warm water rinse before breakfast since mouth tends to dry out while sleeping.
The benefit of brushing (and you should be flossing!) is to brush away all the food and plaque that can build up on and in between your teeth. If you brush your teeth before you eat, your breakfast will remain in your teeth and cause bacteria to build up. The bacteria is what causes cavities! So brush (and floss!) after you eat!