The importance of the last stanza is made evident by the tense shift. The action changes from verbs like "Swung," "tossed" and "shrieked" to "is sitting." This decent into chaos for the poem's speaker has become something that is happening, not happened.
This tense shift makes the poem more frightening in that, up to the last stanza, the entire narrative seemed something that had happened long before, perhaps around the time the speaker lost his loved one named "Lenore." But when the speaker shifts to present tense, it's clear that this personal chaos will not end. With this last stanza, Poe makes sure the reader knows that this ongoing hopelessness is more frightening than a creepy tale about a bird who seems supernatural.
In addition, the stanza includes some intense imagery that suggests the speaker's life is chaos. The raven has "eyes all the seeming of a demon's" and the light from outside casts an eerie "shadow on the floor." Then the speaker compares himself to this shadow. He sees his "soul from out that shadow that lies floating on the floor."
Overall, Poe constructs the poem in a way that the last stanza serves as a shock. He uses the tense shift and some intense imagery to show that personal chaos is more frightening than a creepy story.