"Why did the Bible include four gospels?"
"Why do the gospels disagree?"
"Why didn't God just give us one gospel, like He gave us one account on creation?"
These related questions come to me from Bible students of all ages and maturity, in varying forms of course. The "lack of agreement" question is sometimes couched in language that lets me know the person doesn't want to say that there must be a mistake in there, but they wonder if there is.
The uniquenesses, as well as the similarities of the four gospels is one of the ways that we see how a book written by inspired men is clearly human as well as divine. In fact, each book advances our understanding. Let me seek to show you how (briefly) and then recommend a tool that I greatly enjoy for your own use.
First, the four books give us four perspectives on one story. Matthew, likely the tax collector also called Levi, writes with a clear understanding of the Hebrew Bible and how Jesus fulfilled prophecy--the book brims with quotations cited as being fulfilled. That is fascinating when you think that Matthew would have been considered a traitor to the Jewish people as a Roman tax collector. I can't wait to get to Heaven and hear how this man knew so much Scripture, but also whether he had been wrestling with the truth while outwardly having cut himself off from it! He writes with Jewish audiences in mind. Mark is a man of action, and his gospel could leave you out of breath with its quick pace--much like that of his mentor, Peter. Luke is a doctor writing to help convince/educate someone of significance about the truths about Jesus in a factually precise way (he even uses medical terms). His book has a sequel--Acts. And John, writing much later decides to pick up on much material that the earlier three gospels did not cover to give us a very personal glimpse of Jesus. Each writer even orders their material a bit differently to bring out a different aspect or emphasis in the story.
Second, they provide four complementary, not contradictory witnesses to truth. Now, some argue that they contradict each other, but it is more like witnesses to a car crash standing on different corners--each tells what he sees or knows, without contradicting the other. For example when blind Bartimaeus is healed, Matthew tells us there were actually two blind men together (Matthew 20:29-34), while Mark and Luke only mention Mr. B (Mark 10:46-52, Luke 18:35-43). But that isn't a contradiction, just an added detail. That story has another problem detail--Matthew and Mark say Jesus was going out of Jericho when it happened, while Luke says he was going in. Which is it? The answer is, "both." Jericho had been burned down, rebuilt a short ways away, and then the original site was rebuilt as well--both were called "Jericho." So, Jesus was probably between the old and the new site when the miracle takes place. Thanks to history and archeology, we can answer that fairly easily. Comparing the gospels gives us a rich story--or as one writer calls them, "The Life of Christ in Stereo!"
Third, each gospel has some material that we would lack if that gospel wasn't written and preserved. Even with three gospels paralleling each other (Matthew, Mark, and Luke are called the "synoptic" gospels--a term that means from the same "eye" or perspective), there are precious stories and important accounts we would be missing without each one. Matthew is our only source about the wise men. Luke gives us the births of John the Baptist and Jesus, the story of Bethlehem and the shepherds, and the prodigal son. Mark has four miracles not recorded elsewhere, and of course the story of a young man fleeing without his robe when caught in the Garden with Jesus--probably an autobiographical note giving us Mark's closeness with the Lord and the disciples. What would we do without John 3:16, or the Good Shepherd, the "I am" sayings, and so much more that only comes from John?
Yes, think of them as a Gospel Quartet--Mark sings tenor--that part with all the high notes. Luke sings lead--the thread that holds it all together. Matthew is the baritone, covering some of the same ground as the lead but then moving into parallel paths. And anchoring it all with that solid line on the bottom, John sings bass--hitting those resonating notes of God's love that seem to hold it all together and in tune.
Now, let me finish by telling you about I tool I use regularly when studying the Gospels. It is a book called a harmony of the Gospels (there's another musical idea!). It takes all four gospels and puts them in columns next to each other in chronological order--meaning some passages aren't in the order we find them in their gospel, but in the order in which they most likely occurred. You can read them side by side and see how each tells the story--sometimes word for word, and sometimes differently. Mine is Harmony of the Gospels, by Robert Thomas and Stan Gundry. There is a version in the New International Version and one in the New American Standard Bible (which is the one I have), and you can see it.
I hope that this brief encouragement might cause you to appreciate the gospels more, and perhaps investigate a very helpful tool!