I can spot the rhetorical devices in a passage, but what do I do then?

It is very good that you can already spot them, that in itself is not always obvious or easy! The important thing is to then explain their effect. What do they add to the passage? Look at the context and the point that the author is trying to get across. If you ask yourself why  - why has the author put this in? It will help you understand the sifinificance of the rhetorical devices more easily. There is always a reason for a special positioning of a word. Postponement or promotion may add surprise or humour to the passage, enjambement may reflect and add to a fast-paced description within a passage, while a tricolon acts as a persuasive technique.

For example, if you are reading a sad poem and you notice an alliteration of the letter 'v', remembering that latin poetry was spoken out loud, this would then add a slight wailing sound, that would add to the sadness of the passage.