I'm baffled by this as well. But I've seen a few UK folks do it because they have only one sink on the counter and they use the bowl beside the sink to rinse the dishes clean after they've washed them in the sink. Which makes sense. When I was little, we had two sinks next to one another--one for washing and one for rinsing. I've only got the one now and don't use all that much water, so I do the washing and the rinsing at the same time.