Why use simile?

The initial two similes of the poem, and much of the rest
of the speaker’s language, are commonplace. The speaker is obviously male, an energetic
and fanciful man who does not mind exaggerating a bit in the interests of wooing his
sweetheart and impressing his listeners. His situation is that he is declaring his love
for his “bonnie lass” (line 5), vowing his love’s continuation, bidding her good bye,
and promising to return. The charm of the poem lies in its evident sincerity and
boldness, even if the language is unexceptional. This situation is like that in Donne’s
“A Valediction: Forbidding Mourning”, except that Burns’s speaker does not exhibit the
wit or sophistication of Donne’s speaker.