It is a licensing mechanism, a list of ink formulas, and (most importantly) an entire eco-system of ink swatches and sample books --- a designer can get a customer to agree to a colour, tear off a pair of ink chips, give one to the customer and one to the printer, and the expectation is that the printed piece will _exactly_ match the chips.

There are lots of alternatives --- the Glass Container Manufacturing Institute has a set of swatches which are used for packaging which the industry has agreed on.

That said, there isn't much spot colour work left, and most of what there is, is brand-specific, or done for budgetary reasons (long runs w/ just two colours).