Why does the poem refer to the hammer and the anvil?

In "The Tyger," the speaker asks the tiger who or what could have made him. Noting the phrase "immortal hand," the suggestion is that the speaker is asking the tiger what kind of god would create such a fearsome creature. In Blake's companion poem, "The Lamb," he also asks the Lamb "who made thee?" In this poem, the speaker heaps praise upon the Lamb, a reference to Christ. 

In the "Tyger," the speaker asks "Did...

In "The Tyger," the speaker asks the tiger who or what could have made him. Noting the phrase "immortal hand," the suggestion is that the speaker is asking the tiger what kind of god would create such a fearsome creature. In Blake's companion poem, "The Lamb," he also asks the Lamb "who made thee?" In this poem, the speaker heaps praise upon the Lamb, a reference to Christ. 

In the "Tyger," the speaker asks "Did he who made the Lamb make thee?" In other words, the speaker is befuddled that a god who could make something so gentle (Lamb) could also make something so ferocious. One could construe a metaphor from this. If God can make such beautiful and peaceful things then why does he allow destructive things to exist as well. This is a repeated theme in some of Blake's other poems: that of oppositions. 

The speaker uses the hammer and anvil as a way to reference the work of a blacksmith. Just as the blacksmith creates hard, sometimes dangerous objects with the hammer and anvil, some "immortal hand" has forged a dangerous creature: the tiger. Blake uses blacksmith imagery of fire and steel to highlight the tiger's strength and ferocity.