at the cost of dirtying loads more little bowls

Little bowls are very easy to clean, especially if they just have something like chopped veggies in them.

If you dice your carrots while your onions are sweating, you don’t have time to throw the onion skins etc. in the garbage, or to put any unused onion in the fridge, etc. So, the mess just keeps expanding with each ingredient. And, the worst part is that as soon as you’re done, you want to serve the meal while it’s hot, so all the mess stays out on the counter until after dinner.

If you’re doing everything in one pot, that doesn’t matter too much because you only have one difficult thing to clean, the pot. But, if you’re making a meal with multiple pots and pans, and they’re all sitting dirty while you’re eating, it’s much more of a pain to clean them up after dinner. If you prep everything in advance, you might have a chance to give the pasta pot a rinse while the stuff in the frying pan needs a couple more minutes.