Where Elisa is trapped, the tinker is free. She's a housewife, confined to her house and garden. The tinker roams year round, and the road is his home. What's significant about the tinker's being a foil for Elisa is that he shows her (and us readers) just what she's been denied in her life. Elisa, like the tinker, could be free. But she isn't.
Henry is a blundering, well-meaning, but ultimately unsatisfying partner for Elisa. The tinker, on the other hand, piques Elisa's curiosity and sparks a conversation that we get the feeling Elisa hasn't experienced in a while. Whether he means to or not, whether he's aware of it or not, the tinker comes close to providing Elisa fulfillment that she lacks in her relationship with her husband.