There are many events that are narrated exclusively by Matthew. If Matthew hadn't had his own personal knowledge of the events related to Jesus' ministry, he wouldn't have been able to narrate things that aren't contained neither in Mark's nor in Luke's gospels. Also, according to the writings of the early Church fathers, Mark was one of the apostle Peter's closest companions. Since Peter was closer to Jesus than the apostle Matthew, perhaps Peter (and hence Mark) knew more about Jesus' ministry than Matthew. Hence, that explains why Matthew would have wanted to take some passages from Mark's gospel. Also, anyone who has at least a little bit of experience in writing knows that it is awfully difficult to describe something on paper, even if you are an eyewitness to the events that you are narrating. Hence, perhaps Matthew copied passages from Mark's gospel simply because he liked the way in which Mark had narrated the events, and since he knew that what Mark had written was true, he didn't feel that he was in the obligation to change it. The authorship of Matthew's gospel is attributed to the apostle Matthew based on the writings of the early church fathers, such as Papias and Eusebius, and on the fact that the earliest manuscripts contain the title "Gospel according to Matthew".