In the Preface to Lyrical Ballads, Wordsworth wanted to express his theory of poetry. The Preface is therefore a justification of that theory and of the themes and styles of the poems in Lyrical Ballads.
One aspect of this theory was to use themes about common life (usually in rural environments and situations involving a connection to nature). Thus, Wordsworth wanted to explore how one could attain profound truths and sublime emotional experiences via the imagination. In other words, this process is about understanding the extraordinary while experiencing the ordinary.
Poetry is to be created out of these extraordinary/ordinary experiences. Poetry will be the spontaneous overflow of emotion reflected in tranquility. The poet has an experience and, reflecting on it later, can arrive at a deeper understanding about that experience and about the act of reflection. The process of experience/feeling and reflection is not just a method for poetic creation; it is also Wordsworth's recommended method for experience in general.
Wordsworth wanted the style of Lyrical Ballads to stick with the common life theme. He proposed to avoid personification and traditional poetic diction, favoring instead more common (natural) language of people. In a sense, focusing on feeling (lyrical) more than poetic form (i.e., a ballad), Wordsworth shifts the focus from form to content. Although he was attempting a less formalistic poetry in favor of a more natural (even more prose-like) poetry, he did note that verse was the best form for conveying strong emotional content.
I might perhaps include all which it is necessaryto say upon this subject by affirming, what few persons will deny, that, of two descriptions, either of passions, manners, or characters, each of them equally well executed, the one in prose and the other in verse, the verse will be read a hundred times where the prose is read once.
One could easily argue that a poem (or a song) has a longer life in the memory than a passage from a work of prose (i.e. a novel). This isn't just because a poem tends to be a shorter work. It's also because of the cadence and rhythm, natural mnemonic devices. In depicting poems about realistic, common people in rustic environments, Wordsworth was rejecting the poetry of the past which tended to treat kings, queens, and heroes in an overly regimented style. For Wordsworth, real people were more relevant. More to the point, Wordsworth believed that sublime emotions can be discovered in the experience and reflection of common experiences.
In other words, it can be inspiring to identify extraordinary virtue in a poem about an extraordinary hero whose exploits are unbelievable to the point of being legendary. Wordsworth supposed that (his main concept) it would also be inspiring, more relevant, and more rewarding to identify extraordinary virtue in a poem about ordinary life.