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The reason this happens is to prevent counterfeiting money.
From Snopes:
Do Household Printers Leave an Invisible Tracking Code on All Your Printed Documents?
Mostly true.
What's True
Numerous brands of color laser printers leave coded metadata in barely perceptible yellow dots that can be used to trace a printed document to its source, a feature originally intended as a deterrent to counterfeiting currency with laser printers.
What's Undetermined
While a majority of laser printers are designed to produce this secret metadata, it is unclear exactly how many printing companies and models employ the technology.
Sure. That black and white hundred dollar bill I printed surely won't pass scrutiny with the secret yellow dots.
I didn't say it made logical sense.
The (US) law was passed: "All colour printers must include identifying metadata on every print". So the printer manufactures went...sure, ok.
The job of the government, pass dumb laws to prevent imaginary crimes, while the CIA secretly bangs your mum