Rhyme and Reason
As their names, which play off a well-known idiom, imply, the two imprisoned princesses represent wisdom and common sense. The fact that they were found orphaned in the King of Wisdom's garden suggests that they literally were the fruit of wisdom itself. The princesses, famous for their ability to settle disputes fairly, provide an important counterpart to the knowledge and intelligence of Azaz and the Mathemagician. Milo's quest to return them is, therefore, both a quest to gain wisdom himself (through the lessons he learns during his journey) and to return wisdom to the land (by returning the princesses).
Each gift represents the lesson Milo learned from the character who gave it. During his travails in the Mountains of Ignorance, Milo is able to use these gifts as a way of implementing the knowledge he has gained. Alec Bings, for example, teaches Milo about perspective and gives him a telescope. When Milo is frightened by what he cannot see, he uses the telescope to change perspective and conquer his fear.
In addition to being a vital character who contributes to the storyline, Tock also plays an important symbolic role: he represents the wise use of time. Tock is the only teacher-character in the Lands Beyond who does not give Milo a gift after teaching him something, because Tock's company itself is the gift. He constantly reminds Milo to make the most of his time, helping him to defeat the boredom that defined his life before coming to the Lands Beyond.