We see that as we go from Mark to John the story of the baptism is reshaped to suit the later needs of the community. Christians may have found the implications of the baptism of Jesus to be at tension with the later claims about Jesus' divinity. The way to avoid such tension was to modify the tradition. We get a better understanding of what the authors of the Gospels are saying from this insight into the growing tradition. It helps us to see the various stages of early Christian apologetics at work.
Historical criticism is the fifth type to consider here.
This area of study raises questions having to do with authorship, date, and place of composition of the documents. Most of the Bible would have little meaning for us unless we knew who wrote what when and where. Often we do not have all of this information, but knowing what information we lack also helps us better than if we had not raised the questions in the first place. Knowing, for example, that Hebrews was not written by Paul now leaves us powerless to determine who wrote it. Yet the knowledge that Paul was not the author reduces our chances of misunderstanding Paul and his message which is to be found in his own letters.