What is the significance of the exclamation at the end of the poem the voice you hear when you read silently?

Depending on what precedes the exclamation, it could represent a change in pitch, tone and volume (if you're surprised, you're more likely to be louder with a higher pitch).

jefferson introduced the idea that one does not change a government for "light" causes because he wanted to emphasize the seriousness of colonists' reasons for declaring independence.