Because your body naturally produces things you need to clean, it's like 75% of the reason you do it. Eating stuff just speeds up the production of bacteria. Give your breath a smell as you wake up in the morning before you eat anything, it reeks.

You should brush your teeth in the morning whether you eat breakfast or not.

Because the bacteria in your mouth thrive in dry conditions, and a majority of sleepers are mouth breathers. The bacteria in your mouth are what destroy your teeth and give you bad breath.

Edit: I just saw you said Yes to after breakfast. I don't know if it matters when, just don't make night time be the only time you brush.

For me it started with not breathing well through my noses so my mouth would taste awful when I woke up from being all dry and weird. Then it became an excuse to get my ass out of bed and moving.