CMYK is the colour model for print (cyan, magenta, yellow and black, mixed as inks on paper); RGB is the model for screens (red, green and blue, mixed as light). The same design needs values in both.
The difference comes down to physics. A screen emits light and builds colours from it (RGB, an additive model), while print reflects light off inks laid on paper (CMYK, a subtractive model). The achievable colour ranges don't overlap: vivid, glowing colours from the screen – especially bright greens, oranges and blues – come out noticeably calmer in print.
In practice a brand palette is defined in several colour spaces from the start: HEX/RGB for screens, CMYK for print, and for key brand colours additionally Pantone – inks mixed to a swatch standard, repeatable regardless of the printer.
Warning sign: if the brand book contains only HEX values, the guessing starts with the first business card or leaflet, and the brand colour will look different on every material.