Below is a photo of a hot water tank on the left, drain pan underneath it and mop sink on the right. The purpose of the drain pan is to capture any leaks from the hot water tank and drain it away, in this case to the mop sink.
So then what’s wrong with this photo?
The problem is that water flows downhill and to the path of least resistance. In this case if the hot water tank leaks it will fill the drain pan and the water will overflow flooding the space. Water will not flow up against gravity into the white piping connecting the drain pan and mop sink to drain away.