In his Preface to Lyrical Ballads, Wordsworth states that the purpose of his poems is to tell the stories of common people in the simple language of the common man, or as he puts it:

The principal object, then, proposed in these Poems was to choose incidents and situations from common life, and to relate or describe them, throughout, as far as was possible in a selection of language really used by men.

In contrast to what he calls "deluges of idle and extravagant stories in verse, [and] ... degrading thirst after outrageous stimulation," Wordsworth states that "the Poems in these volumes will be found distinguished at least by one mark of difference, that each of them has a worthy purpose." Wordsworth does mean to offer pleasure, however—not an idle, frivolous pleasure, but the pleasure of moral elevation. 

The purpose is to "communicate the primary laws of our nature," by which Wordsworth means he hopes to communicate the core of who humans are as emotional beings at their best: he is a romantic poet, meaning he is looking primarily for the good in people, even in their suffering. He chooses to focus on simple, rural folk, he says, because the plainness of their lives makes it easier to locate and express the basic feelings of men and women:

because such men hourly communicate with the best objects from which the best part of language is originally derived...being less under the influence of social vanity, they convey their feelings and notions in simple and unelaborated expressions. 

There is no layer of sophistication to peel away with common people, and while he says he modifies their language ("purified indeed from what appear to be its real defects"), he finds a directness and simplicity in their expression that helps to convey the emotions through which he hopes to touch the reader's heart. By recollecting in tranquility the "spontaneous overflow of emotion," a task which he understands as the chief mission of the poet, Wordsworth hopes to convey the best and truest in humankind in its interaction with the natural world.

The most often quoted line in the Preface is ""a spontaneous overflow of powerful emotion, recollected in tranquility."  Their function, then, is to communicate a personal experience, one experienced in Nature and bringing about an apotheosis (an awakening, a shifting to a more comprehensible state of understanding the way the world works) to the reader, in such a way as to reproduce that overflow of emotion into words.  Shelley's Ode To a Skylark is as pure an example as any.  Before The Lyrical Ballads were published, 18 c. poetry was more cerebral, more intellectual and social; the Lyrical Ballads went to Nature for inspiration.  The "Preface" introduced the idea of moving beyond the knowable to the unknowable.  Wordsworth's "Ode: Intimations of Immortality" centers the new poetic movement: "Our birth is but a sleep and a forgetting." The words "spontaneous overflow" point to the feelings not controlled or monitored by Reason; but the key point is that the experience must be "recollected in tranquility," after the emotional excitement has died down.