A common theme that I see in both the novel and the poem is the need for solitude - that we all need time alone with our own thoughts to figure out who we really are, and that once we are able to do this, then we can be of help to others.
In Fahrenheit 451, when Montag goes to see Faber, Faber tells him there are three things basically missing from society: "Number One: quality of information. Number Two: leisure to digest it. Number Three: the right to carry out actions based on what we learn from the interaction of the first two." That quality of information can come from books, he explains, but that is not the only possible course.
Montag's society keeps people from not only having that information, though - it also keeps them from truly having leisure time because of the constant barrage of surrounding noise at home (with the TV walls), on the train (with the blaring ads), and with the many loud forms of entertainment enjoyed by his society. Montag doesn't initially realize how this noise has impacted him, but when he tries to read and retain what he is reading in "The Sand and the Sieve" section, he realizes that he can't maintain his focus and is desperate to find a way to remember what hs is reading.
In the poem, Larkin states:
When I was a child, I thought,
Casually, that solitude
Never needed to be sought.
Something everybody had,
Like nakedness, it lay at hand,
Not specially right or specially wrong,
A plentiful and obvious thing
Not at all hard to understand.
He took solitude for granted, but as an adult, he realizes that solitude can often be hard to find. He also realizes, though, that it is in community with others that our character is truly tested and developed. We have the opportunity to be kind, virtuous, giving loving - or not. By ourselves, we cannot truly practice any of these things.
It is not until Faber is later floating down the river after making his escape that he has the leisure time to "digest" all that he has learned, and there he makes the pledge that he will never burn again. This time of leisure, or solitude, helps him to determine what he believes is truly important, and as a result, after the bombs drop and destroy the city, he is ready to practice "virtue" and "love" toward society.
Montag can come out of his shell like the poem's "simple snail" and unfold/emerge as a man with understanding and convictions. He can help in the process of reintroducing intelligent thought and rebuilding the city, hopefully in a better form. He can act on what he gained from this time of solitude and be "virtuous" in his dealings with others, as the poem says people need to be.