Can anyone explain to me the functional differences between H20.5 (and prior's) 'treat as light source', and H21's Volume lights?

Treat as light source gave us Light AOVs... as far as I KNOW light sampling params also affect 'treat as light source' - are the benefits of a volume light simply utilitarian (light linking, visibility 'as a light' in the scene graph), or are there other render sampling benefits aside that weren't available when using 'treat as light source'?

edit: I guess also Treat as Light Source is also karma-specific...