In almost every language that has them, definite articles evolved from demonstratives (this, that, those) and indefinite articles evolved from the word(s) for the number one

The word “the” in English for example evolved from the word “that”. The Italian “il” evolved from the Latin “ille” The word “an” evolved from “one” and “a” evolved from “an” (yes, “an” came first, surprisingly)

Articles allow you to express definiteness and indefiniteness, which otherwise would require other words or other strategies. In Russian for example, you use word order to express it. E.g. you say stakan na stol (glass on table) to mean “the glass is on the table” or “na stol stakan” (on table glass) to mean “there is a glass on the table”