Because English grammar is so complex, and because a word s part of speech is not always clear, readers of English look for clues to help them identify a word s function in the sentence.
Articles are markers that help readers identify nouns.
The question doesn't make any sense. Definite vs. Indefinite A and AN signal that the noun that it modifies in your sentence is indefinite, that it refers to any member of a group. THE signals that the noun that it modifies in your sentence is definite, that it refers to a particular member of a group. Subsequent Mention A or AN is used to introduce a noun when it is mentioned for the first time in a piece of writing. THE is used afterward each time you mention that same noun.