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.

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

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