If Mathew was an eyewitness, why would he need to use Mark?

The Gospel of Mark is essentially the memoirs of the Apostle Peter. Early church fathers, such as Papias (c. 125 AD), recorded that Mark served as Peter’s interpreter and wrote down his preaching accurately.

Since Mark represented the testimony of Peter, the "lead" apostle and member of Jesus’ inner circle, it would be natural for Matthew to use Peter’s established narrative as a primary, authoritative framework for his account.

Matthew wasn't just recording facts; he was editing and expanding the story for a specific audience: Jewish Christians.

While using Mark’s narrative, Matthew added genealogies, specific Old Testament fulfillments, and discussions of Jewish Law that Mark (writing for a Roman/Gentile audience) had omitted. This practice of using a source but adapting it for a new context was a standard and respected literary technique in the 1st century.

Summary: Even if Matthew was an eyewitness, utilizing Mark's Gospel allowed him to build upon the authoritative testimony of Peter while dedicating more space to the specific teachings and Messianic proofs necessary for his Jewish-Christian readers.

Note: Some scholars argue that the similarities between the two aren't necessarily the result of one copying the other in the modern sense. Instead, they may reflect a stabilized oral tradition. Since the apostles preached the same stories and teachings in synagogues for decades before committing them to parchment, the wording would have become fixed through repetition. The overlap between Matthew and Mark may simply reflect this shared, polished oral testimony.