Black (K) was added to the system for both practical and technical reasons. While theoretically unnecessary (since cyan, magenta, and yellow can create dark colors), black ink reduces ink consumption, improves shadow detail, and creates deeper blacks than CMY alone can achieve. It also dries faster than layered colors and provides crucial contrast in printed images.
The 'K' designation dates back to early four-color printing processes, where black served as the "key plate" for image registration and detail. The black key plate provided the outline and shadows, almost like a detailed sketch, and then the other colors (cyan, magenta, and yellow) would be carefully aligned with this black outline to fill in the color. This was essential for sharp, properly aligned prints. Today's digital printers don't use physical plates anymore, but we still use the term K for black ink