Why use a pulley?
Scholars tend to say that the author uses the metaphor of a pulley to talk about the forces that pull people up and down at the same time. In a pulley, a force that is pulling down, actually lifts something up at the same time. In this poem, the speaker is arguing that the same thing happens with people.
He is saying that it is the forces that pull us down to earth that also lift us to heaven. The stresses of every day life are what pull us down towards the earth. At the same time, however, they are the things that make us understand that we need God and therefore they pull us up towards heaven as well.
The dominant image of the pulley is unusual and ingenious, but it is also brilliantly right. When we realize that pulleys are mechanical devices, that they hold exceedingly firm, and that they sustain and lift great weights, we may see the appropriateness of the image. In this poem, the pulley is composed of the human need to rest and withdraw. Thus, tiredness and restlessness are the means by which God ultimately draws human beings to Him or Her. Though not overtly religious, it is more of a spiritual balance between the weight of our existence with the weight of our humanity.