Symbolism, Imagery, Allegory
Gansey, Barrington, and Neeve are all on a singular quest in The Raven Boys: to find the ley lines, wake them, and receive a great favor. For the characters, the ley lines represent what they desire most in their lives. By seeing what they are willing to do and what they would ask for, we see their deepest, darkest desires:
Whoever controlled the ley line would be more than rich. Whoever controlled the ley line would be something that the other Aglionby boys could only hope to aspire to. (18.13)
Barrington Whelk wants to control the ley lines so that he can have all the power, and so everyone will fear and admire him again. Neeve wants to ask the ley lines for power as well, so that she can become the most powerful psychic around.
Gansey, on the other hand, isn't a power hungry villain. Instead, he just wants to use the ley lines to find Glendower so that he can carry out his life's goal. He doesn't even know what he would ask for besides that; he just wants to see why it is his destiny to find Glendower, since he was allowed to cheat death once in order to complete this quest.
The ley lines, then, represent the paths the different characters are on, as well as revealing important clues to who they are.