Water and compound are added into the machine to facilitate the cutting process and remove any debris coming from the media, parts and any additives so the process stays clean, the media can keep cutting and doing its work as it should do and to protect the parts being processed (i.e. with a rust inhibitor if corrosive).
Abrasive media (for deburring, radiusing, blending machining marks, etc…) needs to have an ‘open face‘ to work as it should. This is where you can feel the abrasive particles when dragging your finger nail over the surface of the media (like a file).
As most media (ceramic and/or plastic) wear, the level in your machine can drop significantly over time. As mass finishing processes work by covering the part with the media and the media moving over the edges to create the deburring, radiusing, etc.. it is important a ratio of media : parts is retained.