With Spotify images can be converted into a limited number of spot colors. This feature is probably only useful for a few users, but if this requirement exists then Spotify is certainly extremely helpful.
Some printing processes do not use CMYK process inks at all, instead only spot inks are available, and sometimes even a relatively small number. Furthermore, in some cases half-toning does not work at all, and thus it may not be feasible to overprint spots with each other to achieve mixed ink colors.
This may be due to the way the printing process works, but could also go back to cost considerations, for example when imprints on small give-away items must be as inexpensive as possible and thus use only two spot colors.
For such scenarios, print ready PDFs should only use a specific number of spot colors, and always at a 100% tint value.
You can find more information in the Callas help pages: