The purpose of WaterPuddle class in GameVolumes is to create a simple shape that outlines a puddle that could be created from decals.

Note that this GameVolume does not render water. It is purely a shape object.

While you could technically create puddles out of water volumes, it is of course much cheaper to use decals.

The problem you then face is that you lose support for special environmental effects, such as firing an electrified arrow into a puddle of water doesn't quite work when it's just a simple decal.

Surrounding the decal with this GameVolume will allow the engine to understand that the designer wants this to be a puddle and acts accordingly.

There is a parameter "ThisIsAPuddle" on this entity. Please note that this is a dummy parameter. It is not actually used and does not matter whether it's set to true or false.