In Lord Byron's " she walks in beauty," the speaker describes daylight as overly bright and showy, or

In the poem, Byron writes that daylight is "gaudy."

He does this to prove that daylight is overly bright and showy. His love is not like daylight - - she is not bright and showy.

That's why she walks like the night - - she does not need to flaunt her beauty. Her beauty is natural.