Personification allows this poem's iris to speak directly to the reader, and hints that the flower's rebirth might also be an image of something that happens to humans.
This iris talks to its readers in the first person, remembering its experience of death and resurrection with a lot of detail and feeling. "It is terrible," it recalls, to be a "consciousness / buried in the dark earth." Moments like this make this poem work differently than other poems that use flowers as a symbol of rebirth. It's one thing for a poet to look at a flower and think, "How hopeful, flowers always come back in the spring!" It's quite another to imagine going through the harrowing, frightening experience of death while still "conscious[]," on the way to the spring. By allowing the iris to speak of its ordeal, personification allows the poem to explore the real terror and pain of undergoing a rebirth.
The iris's personification also suggests that people go through similar cycles of flowering and death, over and over. That might be true in a metaphorical sense: people endure grim periods of despair, feeling like they're "buried in the dark earth," only to emerge into the sunlight again. But perhaps this iris even offers hope that this is literally true: that death isn't the end, just a stage in an ongoing process of life.
Readers might even interpret this personified iris as the voice of the poet herself! When the iris talks to the reader, it seems to know that what the reader "fear[s]" is being conscious but mute—being "a soul and unable / to speak." This fear of being unable to speak might suggest a particularly poetic difficulty: the feeling of going through a period so dark that it's impossible even to write. The personified iris might speak for a poet who has suffered, but emerged to find her "voice"—a voice she'll use to share what she's discovered about life and death.
Personification thus makes the iris's experience seem deeply human.