But as you say, if B and C are already in food, why the need for an empty stomach?
It depends. In an ideal world one would get all nutrients from foods, and there would be no toxins in it either. Which isn't the case with our industrialized world and many ills rapidly increasing due to that.
So the recommentation for example with fat-solubles to take it with fat is to increase and optimize absorbtion in the circumstance of lack.
With watersolubles this is less of an issue. However, specially Vitamin B12 is very poorly absorbed through the stomach, and there are many contitions including age which drastically worsen it even more. Therefore I would prefer sublingual longzenges in this particular case.
Then there are the many cases of specific deficiencies, where only pharmacological therapeutic doses could make a tend, and therefore absorbtion to be increased additionally beneficial.
Amino acids too compete in absorbtion, therefore if one wants specific therapeutic effects from one of them, better to take in isolation.
Some minerals might not be well tolerated on empty stomach and are then better taken with food.
Vitamin C is on one hand an essential vitamin in minute amounts, but also a great therapeutic for certain diseases at mega-doses. In fact my favorite, since it help me overcome a walking-disabilty from a stenosis at my abdominal aorta.
So with no disease you'll be fine taking vitamins, minerals and amino-acids with food. I took vitamin C always 20 minutes before meals for better absorbtion, in average 25 gram per day for the last 13 years. I could of course also just doubled that dose to make up for less absorbtion when taken with food.