In "Methought I Saw my Late Espoused Saint" the speaker sees a vision of his dead wife. She literally appears to him (okay, so it's maybe in a dream), which means that appearances in general play a big role here. The most important aspect of the vision of this guy's wife is not her physical appearance, but the good, spiritual qualities the speaker is able to perceive in his wife. Because she's dead and in heaven, the idea here is that she's in a place where the barrier between body and soul collapses so her appearance seamlessly communicates the state of her soul. Ah, so that's why she's a saint—she's beautiful inside and out.
We have no idea what this lady really looks like, and it's not at all important. That's because she's a figment of the speaker's imagination, and therefore doesn't actually look like anything at all.
The description of the vision in "Methought I Saw…" emphasizes its spiritual, rather than physical, qualities. Her spiritual appearances are much more important than her physical ones.