The articles have to do with the category of Determination (definiteness/indefiniteness).
Causes for Rise of Articles:
1. In OE the there were two declensions of adjectives – strong (definite) and weak (indefinite) – and the inflections of these declensions indicated whether the noun that followed the adjective was definite or indefinite. At the end of the ME Period the declensions of the Adjective disappeared and there was a necessity to find another way to indicate the definiteness/indefiniteness of a noun. Thus the articles appeared.
2. In OE the word-order was free because inflections were employed to show the relations of the words in a sentence. In ME and NE the majority of the inflections disappeared and the word-order became fixed. This meant that the first place in a sentence was usually occupied by the theme (information already known à marked with the definite article) and the second place – by the rheme (new information à marked with the indefinite article).