Roman numerals are part of the common knowledge in Western cultures for representing numbers. That's why it's valuable for children growing up in these societies to learn them. It's the same as the reason we teach children Arabic numerals.
If you don't know how to read Roman numerals, you'll be effectively illiterate—or "innumerate"—whenever you encounter one. Roman numerals function like "italics for numbers": a distinctive, alternative way of writing the same thing. For example, they're used for page numbers in long prefaces to distinguish them from the main page numbers of the book, for items in hierarchies (such as essay outlines) to distinguish different levels, for the roots of musical chords where Arabic numerals denote intervals, for groups in the periodic table, etc. They denote ordinal numbers of monarchs (e.g. Henry VIII), sporting events (Super Bowl LI), yearly formal gatherings, world wars, version numbers, movie sequels, clotting factors in the fibrin cascade, etc. And they represent years or hours in very formal contexts such as stone inscriptions, sundials, and clocks meant to last a long time. When people write Roman numerals, they expect that any member of our society with even a minimal education will understand them. They're even used in slang: a "C-note" is a hundred-dollar bill.
That's what I mean by "common knowledge": knowledge or conventions that are shared by most people in the culture and that are known by most people to be shared. When something is common knowledge, we can depend on other people knowing it, other people knowing that we know it, etc. The classic example of common knowledge is language: without that, we couldn't communicate much at all. Roman numerals are just part of our common language of numbers.
If you're thinking that numeracy, since it relates to mathematics, should be independent of culture, that's an error. By that reasoning, Arabic numerals would also be excluded. From the standpoint of pure mathematics, the fact that the symbol 6 stands for the same abstract quantity that we refer to by the word "six" is an obscure piece of trivia. "Numeracy" is a vague concept, but I understand it to emphasize practical matters, and that includes knowing your culture's conventions for representing numbers—even a less-often-used convention that exists alongside the most common one.
Tangential mathematics and history
Some people argue that Roman numerals should be taught because they illustrate things like multiple ways to represent a number (a kind of flexibility that you need for algebra and anything beyond), or the value of positional notation, or the value of a symbol for zero (since you really come to appreciate something by trying to do without it), or that the C and M stand for the Latin roots of our prefixes centi- and milli-. Roman numerals provide an opportunity to teach all those things, and children may learn them as a happy side-effect even if the teacher doesn't point them out, but the related topics aren't the reason for teaching Roman numerals. Kids need to learn Roman numerals so they can participate in the common knowledge of the adult world—the same reason that nearly everything in elementary school through high school is taught. Notice that if learning the related topics were sufficient reason to teach Roman numerals, then Roman numerals should be taught even in Mongolia.
By the way, the numerals used by the ancient Romans did not exactly follow the conventions that today we call "Roman numerals". They were less standardized and the forms changed over time. For example, one convention represented 500 as IƆ; 18 might be represented as IIXX. More here—but those sorts of things are relatively obscure trivia, not common knowledge. While the Roman numerals taught in elementary school today have deep roots in our past, they are part of our present-day culture.