Hygiene-wise, why would there be a particular need to brush after breakfast if there's no particular need to brush after other meals?

I am addicted to marmite, which I spread on my bread at breakfast, but the taste is so strong that I feel filthy unless I clean it out of my mouth fairly soon afterwards.

I never saw the point of cleaning teeth as soon as I get up, as I couldn't see there'd be very much change from when I cleaned them just before going to bed. But I didn't have a German teacher to tell me about germs building up in my sleep.