Other gospels simplified
If you accept Matthean priority, the question becomes, "Why did Mark and Luke halve people?" It's even simpler than that since Luke usually prefers Mark's text. Robert Dean Luginbill (author and curator of Ichthys.com) argues:
There were two, so Matthew gives two. But the fact of "two" raises questions which a concentration on the more significant "one" in Mark alleviates (getting to the point without complicating things for the new convert: a valid technique with God's apparent stamp of approval). Take the donkey and the colt. Yes there were the two, but of course Jesus rode one. And why two? To fulfill in every respect in a way that would be unmistakable the Zechariah prophecy, an unmistakable sign to all who watched Him ride into Jerusalem that day on a donkey's colt with a second donkey in tow (Zech.9:9; and cf. Gen.49:11). The two mounts are also symbolic of the two advents of our Lord (cf., Rev.19:11; and possibly also of the soon to included gentile portion of the Church added to Israel: Peter was ministering to a gentile audience). Jesus rode into Jerusalem once as the suffering Servant, and at the Second Advent He will come on a white horse as the conquering Messiah (exactly what the crowd who welcomed Him with hosannas was expecting in 33 A.D.). But to include all this in Mark meant explaining the issues involved in interpreting Old Testament prophecy and explaining the expectations of Jewish believers at the time of His crucifixion, both of which were likewise completely foreign to the Roman mind (they still sometimes cause difficulties for new Christians today).
So for shortness, simplicity, and to avoid making issues of things which, while important, were not as important as the basic message of the gospel, Mark was written as it was—the truth put in a way to make truth the issue.
Since I prefer Markan priority, Matthew's account requires a different explanation. One or more of his sources might have experienced the Third Man factor. A number of anecdotes suggest that in times of extreme danger, people occasionally notice a person accompanying their party who is a stranger. T. S. Elliot described an incident of this phenomena (which Sir Ernest Shackleton experienced on South Georgia Island):
Who is the third who walks always beside you?
When I count, there are only you and I together
But when I look ahead up the white road
There is always another one walking beside you
Gliding wrapt in a brown mantle, hooded
I do not know whether a man or a woman
—But who is that on the other side of you?
—The Waste Land, lines 360-366
Perhaps more relevant to the New Testament, Shadrach, Meshach, and Abednego were visited by another person when Nebuchadnezzar tried to immolate them. The term "Third Man" is a bit of a misnomer, since the vision is usually of one extra person in the group. There are spiritual explanations to these visions, but there's also a psychological aspect: in times of great stress, it can be helpful to know that we are supported by other people. That remains true even if we know that those people cannot physically help us.
Unfortunately, with the possible exception of the demoniac(s) of Gadarenes, the situation does not fit the usual profile of the Third Man. There was no particular danger (physically or otherwise) in healing the various blind men and there's no precedent for a "Third donkey". This theory is doubly problematic if you believe (as I do) that Matthew was an eye witness to these events. The curious thing about the Third Man syndrome is that the witness know the extra person was a vision and are not confused about the number of people who were physically there. Since Matthew had access to Mark (and knew that some of his
It's possible that Matthew was enterprising a poetic technique by adding extra people to his accounts. If so, he wrote knowing that some of his readers were familiar with Mark's biography of Jesus and noted especially important incidents by using a numerical ladder. With this understanding, the healing of the blind was so important that Matthew not only doubled the number of people Jesus healed, but doubled the incident itself. Mark 10:46-52 identifies the blind man as Bartimaeus and Matthew seems to have used aspects of his story (especially the title "Son of David") in several places.
We need not conclude that Matthew exaggerated or fabricated the details of his narrative. Since Jesus clearly healed many people of blindness and demon possession, so the healings Mark (and later Matthew) decided to fill out into detailed stories are representative. Jesus might have healed many blind people at Jericho, but Mark only described one. Matthew, therefore can be seen as restoring the scale of the miracles without sacrificing (too much) descriptions of Jesus' personal touch.
Unfortunately, we can't know for certain why Matthew occasionally, used two individuals when Luke and Mark used one. The above is largely speculative. The explanation is substantially simpler if Matthew was the first gospel as it's easier to imagine later accounts simplifying the stories rather than expanding them.