Because the English language uses number, which requires a determiner before a singular countable common noun-- either an article, a demonstrative adjective, or a possessive adjective. You don't have to use 'a car' if you choose instead to use 'the car', 'this/that car', or 'my/his/their/Jesus' car'. Additional adjectives are optional: 'an old car', 'James's stolen car', 'this decrepit car'.
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