Why might an author use irony in a literary work?

to summarize the events and characters
to emphasize important themes
to promote the use of a particular setting
to motivate the reader to use reference materials

faithful servants eumaeus and philaetius:

his ragged vest then drawn aside disclosed the sign conspicuous, and the scar exposed: eager they view'd, with joy they stood amazed with tearful eyes o'er all their master gazed:

these servants know ulysses by the scar he got when he was a child - so when he showed them the mark, they immediately knew this is truly their master and they hugged him, finally starting to believe his words.