Emily Dickinson is famous for her unusual punctuation, especially her signature dashes. She often used dashes as substitutes for commas and periods, and sometimes inserted them in places where there wouldn't normally be a grammatical pause. Occasionally, she inserted "unnecessary" commas as well. As a result, her poems tend to contain tense or dramatic caesuras, and this one is no exception.
Look at the first three lines, for example:
I died for Beauty—but was scarce
Adjusted in the Tomb
When One who died for Truth, was lain
In line 1, a dash takes the place of a more conventional comma, emphasizing the word "Beauty" and setting up a kind of dramatic twist (But suddenly...). In line 3, the comma doesn't technically need to be there, but the pause helps stress the thematically important word "Truth."
Line 7 again emphasizes "Truth," this time with the help of two dashes:
"And I—for Truth—Themself are One—"
Grammatically, only the second caesura is necessary; the dash after "I" just adds drama! Another seemingly gratuitous caesura, also marked by a dash, appears in the final line:
And covered up—Our names—
Notice how the extra punctuation slows the pace of the language. This slowness feels appropriate in a poem about death and eternity, featuring "Moss" creeping over corpses.
The eccentricity of Dickinson's punctuation is also a bit disorienting, making it a good match for the jarring psychological and philosophical territory it explores. When her poetry was first published after her death, editors tried to eliminate these effects, standardizing her capitalization and punctuation (for example, removing weird caesuras like the one in line 12 here). Later editors acknowledged that these effects were important to her art, and printed her language just the way she wrote it.