The ostensible purpose of Montesquieu's Persian Letters is for Usbek and Rica to inform their correspondents of what they have learned through travel. Beneath this, of course, is Montesquieu's satirical intent, which subverts the simple desire to inform, as the noblemen often misunderstand the customs and cultures they experience. For instance, they say that the people of France are so gullible that the King only has to distribute pieces of paper with his picture on them and the natives accept this as money. Here it is the ignorance of the Persians that is satirized (banknotes, of course, originated considerably further East than Persia, in China) but often the Europeans are the target, particularly when the Persians notice similar corruption and frivolity in East and West.
The principal difficulty of learning through travel, explored throughout the book, is this: we understand other cultures by comparing them with our own. If something is entirely different in another culture, therefore, we fail to understand it. If it is the same, it merely reinforces our previous experience. Either way, we learn nothing new.