Strange. I know of one system where the combi heats a tank in the normal
way, and the water heating side is used just for the kitchen. Not sure
what the benefits are, as the owner is as mad as you.

It can be good in a large house to reduce water lead times, particularly if
the boiler is in the kitchen. The hot water cylinder is placed near the
bathroom to provide lots of hot water quickly, whilst the kitchen runs off
the combi, because you'd have to draw off 10 litres of water before anything
came through from the distant bathroom. It also provides drinking hot water
in the kitchen, which is very useful when cooking. I use my hot tap all the
time to fill kettles and pans. (It's a heat bank, not a combi, but still
drinking water).

Christian.