Blake uses stronger elements of cacophony in “The Tyger” and euphony in “The Lamb” to emphasize the different temperaments of the creatures.

Cacophony is a dissonant or unpleasant sound.  “The Tyger” is a harsh poem, demonstrating the ferocity of the beast.  In this line, you can see the force of the alliteration.

What the anvil? what dread grasp 
Dare its deadly terrors clasp? 

The fierce elements are subdued in “The Lamb,” where euphony is emphasized.  This way the poem has a gentler tone and rhythm.  The poem is much sweeter and calmer.

He is called by thy name,
For he calls himself a Lamb: 
He is meek & he is mild, 
He became a little child

The poem is much more mellow, especially in contrast to the roughness of the tiger's poem.  Blake intended the two to be contrasted and compared, and chose the rhythm carefully to mimic the animal's nature.