The synoptic gospels refer to Matthew, Mark, and Luke. These three gospels are very similar in content, as if they were seen with the same eye, hence syn-optic literally means “with one eye”.
There are several theories regarding these three gospels. Most scholars believe they share a common source for certain parts, while individually they also have separate sources. What we do know for sure that they were written for different communities.
Matthew was written by a Jew for a Jewish community. It is likely that Matthew dictated in Aramaic and his scribe wrote it down in Greek. Because of its unique background, it is strong on emphasizing Jesus as the new Moses, fulfilling the Mosaic Law. It also presents Jesus as the Teacher, with three chapters devoted on the Sermon on the Mount like Moses in Deuteronomy.
Mark was written by a Jew for a Greek community. It is possible that the community was under some impending crisis, as Mark’s account seems to always be in a hurry, with the most use of the word “immediately”. This gospel is also the shortest. It can be read in one sitting. Many scholars believe that Mark was in fact the first gospel written, since despite its shortness, the individual stories tend to possess more details that are evident of an earlier telling of the story before they are condensed due to repetition.
Luke was written by a Greek for a Greek community. Being a Greek, more removed from Jewish understanding, Luke tend to focus more on universal charity, as well as the role and power of the Holy Spirit. Luke, being also the author of Acts, also tend to focus more on themes that continues onto the early Church living.
I hope the above synopsis help to illustrate to you why there needed to be three versions of the gospel that seem to be so similar and so redundant. They were written specifically for different communities and they are in fact different in significant ways though anchored on the same message.
They are like three different persons, having individually experienced the same events, went home and told their own experiences to their communities. And isn’t it amazing that they are still so similar with virtually no significant contradictions? That’s why we know these books are inspired by the Holy Spirit.