Identical to the hex version
Hex is RGB written differently, so this is the same conversion with a different input. If you have channels from a canvas or an image sample rather than a string, start here.
Still an approximation
Everything on the HEX to CMYK page applies: the formula is the naive one, print colour depends on ink, paper and profile, and nothing produced here should be sent to a press. It is for understanding, and for filling a field in a mockup.
The practical use
Sampling a colour from a photograph and wanting a rough sense of what it costs in ink — a wash of pale sky is cheap, a saturated orange is not. The four numbers give you that at a glance, and that is about as far as they should be trusted.
Questions
How is this different from HEX to CMYK?+
Only in the input. Hex is RGB written in base 16, so the conversion and all its caveats are identical.
Can I send these numbers to a printer?+
No. The formula is the naive one and ignores ink, paper and profile entirely. Ask the printer for their profile and convert with it.
What is it actually good for?+
Getting a rough sense of what a colour costs in ink — a pale wash is cheap, a saturated orange is not — and filling a field in a mockup.