The water tray holds water that is dumped for flushing the toilet, instead of the flapper that you normally see.
As we all know, flappers can gio bad and leak.
In that regard, the water tray of the Niagara reduces that area of repair.
Not that you are totally home free though. There are still handles that can break, and trays that can crack.
It uses the Fluidmaster repair parts for the fill valve.