How should we use figures of speech in Wordsworth's and Shelley's poems to understand the poems?
The object of figures of speech is to express an idea or experience in a metaphorical, symbolic or other non-literal (figurative) way for the purpose of making the underlying idea or experience more understandable. This may seem contradictory considering how hard it sometimes is to identify figures of speech (also called figurative language) but figures of speech affect our understanding when we read them even when we can't identify or label them.
Let's take an example from Wordsworth's The Ruined Cottage. When you read the following, what sense of meaning do you get from the words?
... four naked walls
That stared upon each other
Most readers who see this think immediately of a room or hut that is completely bare and abandoned. This is exactly correct for this is Wordsworth's first description of the ruined cottage (Wordsworth got his syntax a bit confused here since his description is of the exterior of the cottage and "stared upon each other" usually conjures up the idea interior walls).
I found a ruined house, four naked walls
That stared upon each other
Readers understand this even without being able to identify this as a figure of speech, called personification, that treats inanimate objects (walls) as if they had human emotions, actions and thoughts. So, in one sense, "how" we use figures of speech to understand poems is simply to react to them, as you and others react to the lines above.
In another sense, we use figures of speech to analyze the deeper meanings the poet is attempting to convey. In this case, by recognizing that this quote is personification of the "house," we can analyze the cottage as symbolizing its past owner's life and love. The poem will bear this analysis out as the story of its inhabitants is told from their prosperity to their ruin--and the symbolic cottage's ruin.
Let's apply this to a quick example from Shelly's "When the Lamp Is Shattered." Stanza III tells us his topic is passionate love (though love is a double symbol for (1) the beloved one and for (2) the passion): "Love first leaves the well-built nest." Stanza IV starts:
Its [love's] passions will rock thee
As the storms rock the ravens on high;
These two lines have three figures of speech: personification, metaphor, analogy. If you don't know about figures of speech, you still sense the meaning that love will bitterly break your heart. Analysis of the figures confirms this by showing that love is given the human trait of violent emotion that will shake you as if in violent anger and that it will be compared to how a storm shakes the trees ravens are perching in for protection. We see from analyzing these figures of speech that there will be no protection from the violence of love's passions.
So the way we use figures of speech to understand poetry is by analyzing the figures' deeper, non-literal meanings to deduce the poet's underlying meanings and themes.