Personification helps to characterize the speaker's dream-garden as a vibrant yet tender place. This isn't a still, cold scene but a moving, living one.
For example, the poem describes the "waters" of the stream that passes through the garden as "murmuring." This is a common description for the sound of water, yet it's still evocative: to "murmur" is to speak lowly, so the word suggests both that the waters are talking to the speaker and that the speaker must take care to listen closely to hear what they have to say. Rather than try to assert control, the poem implies that an artist must be quiet, open, and receptive to what the imagination holds.
There's more personification at the end of this stanza, which once again conveys the sweet, tender nature of the earth. The speaker says that the ground
[...] hardly dared to fling
Its green arms round the bosom of the stream,
But kissed it and then fled [...]
The earth here seems like a shy lover who can barely bring themselves to embrace and "kiss[]" their beloved before running away to hide. The personification adds to the whimsical nature of the speaker's dream.
The personification of the next stanza works similarly, as the speaker describes a flower shaking raindrops onto the ground in the wind. This "tall flower" is a child of the earth who "wets [...] Its mother's face" when the wind—also personified as the flower's "playmate"—it hears. Again, the garden is an innocent, delightful place.