Most artists buy these one at a time, months apart, usually because an opportunity showed up and they didn't have the file. A playlist wants a clean version. A sync library wants an instrumental. A promoter wants something to perform over.
Bundling them costs less and means you have them before you need them.
Available for songs we mixed, since these versions are exported from the mix session. If we didn't mix it, we'd need the session or stems first.