I don't have a dishwasher, and I use the "bowl in the sink" method. I've never thought about why, but it's what my parents always did and I just assumed that it was how things were done. It does save on the amount of hot water and washing-up liquid used compared to doing it in the sink, and it also doesn't take so long to get the water to a usable depth.

(But if I only have a few not very dirty items that won't require washing-up liquid to get clean, then I just rinse them under the hot tap and don't use a bowl.)