According to Biber et. al. in the Longman Student Grammar of Spoken and Written English:
For each class of lexical word, there is a major phrase type with an example of that class as the head: noun phrase, verb phrase, adjective phrase, adverb phrase, and prepositional phrase. The head is the principal, obligatory word. In fact, each phrase type can often consist of just one word: the head.
In the previous chapter, the author divides the words, as a grammatical unit, into three major 'families': lexical words, function words and inserts.
As silly as the question is, i'd like to know why do we discuss lexical words, such as nouns/verbs/adjectives/adverbs, as the heads of phrases and out of a number of function words only prepositions serve that role?
I apologize if the question is still unclear and/or inane.