The function of interjection is to express emotions or feelings. Usually it is a word or phrase.
I believe that flashbacks could be used to slow the pace of a story. If you are telling one story, and then decide to introduce a flashback, that would only slow down the story you are already telling.
The irony of the poem is that Ozymandias claims to have done many great works and boasts of his might yet he lies in the middle of a desert, broken with nothing to support his claim of his deeds