Similes help the speaker both to articulate a difficult truth and to accept it. The poem describes a memory that the speaker has carried for eighteen years and that has "something [he] never quite grasped to convey." It has been difficult for him to fully comprehend the meaning of his memory and to convey that meaning, or put it into words. But similes allow him to get at that "something" by approaching it indirectly, saying what it is like even if he can't say exactly what it is.
The central event of the poem is the speaker's son walking away from him after playing his "first game of football." The speaker uses several similes to describe this event, how the boy's figure appeared as he left. In lines 4-5, the speaker says the boy went "drifting away" "like a satellite / Wrenched from its orbit." This simile not only conveys the boy's sense of unfamiliarity and unease in his new environment but also helps explain it: the boy is uncomfortable because he has left the path he ought to be following, as satellites are meant to follow their same orbits.
It is true that, while they are young, children need to be guided along a secure path, usually by their parents. But it is also true that, at a certain point, they need to be allowed to leave that path behind. Another simile helps the speaker see and accept this difficult truth of parenting. In line 12, he describes the same event, the boy's walking away, with the simile of "a winged seed loosened from its parent stem." Unlike a satellite, a seed is meant to leave its old environment behind. If it doesn't find a new territory to grow, where it won't have to compete with the parent plant for resources, it won't survive.
The satellite simile represented the child's departure as unnatural. But the seed helps the speaker understand how it can be natural and necessary for children to leave their old, secure environments, even if they seem painfully insecure as they do so. This simile, then, helps the speaker towards his final conclusion a few lines later, in line 20, that "love is proved in the letting go." If the winged seed needs to be "loosened" in order to grow, then the loving parent must be willing to loosen it.