The speaker's similes help to evoke the terror of confronting the past.
In the poem's first simile, the speaker stands on the doorstep of a former home and hears this disturbing sound:
The Silence—like an Ocean rolled—
And broke against my Ear—
That image creates a mood of painful suspense: it suggests that the speaker is straining to hear any hint of what's going on inside the house, but gathering absolutely nothing. They're left adrift on a vast, oceanic silence that might well swallow them up. Perhaps this image even suggests that all the familiar things the speaker once knew and loved are "drowned" and lost in the silence of the past.
No wonder, then, that opening that ominously silent door proves too frightening for the speaker. They manage to get right up to the point of putting their hand on the "Latch," but at the last moment, they chicken out. Listen to the simile here:
I moved my fingers off, as cautiously as Glass—
There's a potential double meaning here. The speaker might be moving as carefully as if the latch were fragile as glass—or as if their fingers were as fragile as glass. Either way, this simile creates a mood of serious peril: at any moment, the speaker feels, something might shatter.
The poem's final simile reveals a lot about the way the speaker feels as they flee in terror: "like a Thief," the speaker recalls, they "Stole—gasping—from the House." In other words, they leave feeling, not just as if they're running from a threat, but as if they're doing something wrong and shameful in running.
These lines might also contain a subtle pun. Here, when the speaker says they "stole" from the house, they could just mean that they crept away quietly. But if they're stealing away like a thief, perhaps they're also stealing something from the house: making off with an undisturbed memory of how things were before, without confronting how things are now.