The speaker's apostrophe to her faraway husband gives the poem its intimate, loving tone. In much of this poem, which is presented as a letter, the speaker reminisces about her life with her husband, her affectionate voice suggesting that she knows he remembers everything she describes as well as she does.
In one sense, the speaker and her husband have known each other forever: the two of them used to play "about the front gate" when they were children, the speaker "pulling flowers" and the husband-to-be "playing horse" on his "bamboo stilts." In another sense, though, their marriage is still fresh and young. The two, the speaker reports, were married when she was just "fourteen"; she's "sixteen" now, and her initial "bashful[ness]" around her husband has only recently evolved into a passionate love.
Directing her memories of their time together to her husband, the speaker comes across as vulnerable, sincere, and sweet. She's absolutely not hiding anything from him, from her initial shyness and resistance to her current overwhelming longing for him to return.
Apostrophe thus helps to characterize both the speaker and her husband. It's not just the speaker's picture of her husband "dragg[ing his] feet" when he left, but her willingness to write so directly and openly to him, that makes it clear this marriage is a truly loving one.