[colejohnson66]

It's not without purpose though. It does it because printing all four colors ("registration") instead of just black makes for a deeper black (if that makes sense).

However, you don't want to use 100% registration for text as it can easily eat up your ink/toner. A good compromise if you need deeper blacks is 100% K with a bit of CMY (don't know the percentage).

I'm not discarding the idea that printer companies do it so the customer has to buy more ink; that may be somewhat true. However, even high end printers that are designed to last still do this to an extent, so it's not entirely a money grab.

[evadne]

https://en.wikipedia.org/wiki/Rich_black

[michaelt]

If you need some inks other than black to make black, why aren't they in the black ink already?

[colejohnson66]

Sometimes you don't want rich blacks, and it's easier to just use some of the CMY instead of having multiple K rollers.