Engine raw water system

Question, on my DD 8V92, the raw water system pumps water through the inter cooler, fuel cooler, heat exchanger, through a pipe with a T fitting, into the shower head . The T fitting allows water out the side through a valve. What is the purpose. Thank you P.

I have the same set up with my 8V71Ti. I belive is to reduce the water pressure on the showerhead input (?) but it becames handy to know if the impeller of the water pump is primed. But I could be wrong.

In the old days it reduced water in the exhaust system thus reducing back pressure.

Thanks, figured it had to do with back pressure. All the best. Pete