Often it's necessary to study the parallel accounts of an event as recorded in more than one of the Gospels, or to understand when in the course of Jesus' earthly ministry a particular event occured.
This is not intended to replace the Gospels and it's highly encouraged that each Gospel be primarily read and studied as a whole because of the unique message each Gospel writer conveys. This is merely a study aid and, in fact, should be thought of as a work in progress that may get revised from time to time since the exact location of every single event is not an absolute and open to interpretation.
Using Luke as the chronological baseline, the other Gospels have been integrated accordingly. This really highlights the differences in Matthew, which is obviously not chronologically-based but is organized to tell a specific message, as well as John, which tends to be focused more on events such as Passover and Purim rather than geographic places of ministry like the other Gospels.