The benefit of having four gospels instead of one, is like the benefit of having three dimensions instead of two. For the same reason that Hebrew parallelism develops thoughts through synonymous, synthetic, and antithetical verses, God develops the image of Christ through parallel books in the Bible with parallel passages. In the Old Testament we have entire parallel books such as Kings and Chronicles; in the New Testament we have the four gospels, and 2 Peter and Jude. While most scholars subscribing to the historical-critical school of biblical studies would suggest this block parallelism betrays the work of a redactor--or editor--those who see Scripture as the product of divine superintendency should understand it as a form of textual depth perception.
Hence, the benefit of having parallel accounts is that they produce a “parallax” effect; two eyes allow us to see better with depth perception. The same is true with Scripture--and in the case of Christ--God has given us four gospels, so that we may know “the width and length and depth and height” of the love of Christ (Eph 3:18).