D as a functional head
D determines the referential or quantificational properties of its complement noun. In English we have various types of determiner reflecting these properties, e.g.:
Motivation for assuming a D node:
(a) There is a kind of agreement relationship between determiners and nouns. Although evidence for this comes mostly from languages other than English (e.g. case agreement, gender agreement) we can see this in English with number agreement.
D is a functional head because it has abstract grammatical properties (rather than lexical properties), dominating features such as number, gender and case.
(b) Determiners are in complementary distribution with other determiners, i.e. there can be only one determiner in combination with a noun. This is a typical characteristic of heads.
(c) D can combine with a complex nominal constituent, suggesting that it takes a complement.
Under the assumption that D can bear case features possessive DPs can be licensed in the specifier position of DP (e.g. John's in John's friend).