While hunting for a leak in my roof space I found a tank. It has a pool of cold water.

There is a pipe bringing water IN to the top of the tank which is determined by a valve controlled by the ball float.

There is a big pipe high up to drain the water to outside of the house should the ball float valve fail.

There are multiple pipes (you can't see) at the bottom of the tank going multiple directions.

What the hell is this?

Note my hot water cylinder is old and has its outlet at the top and there is nearly zero hot water pressure.