Jesus strongly rebuked His disciples for their lack of spiritual understanding (cf. Isaiah 6:9-10; Jeremiah 5:21; Ezekiel 12:2). In view of the two miraculous feedings they had witnessed, they should have understood who He was. They did "remember" the facts ( Mark 8:19-20), but they did not "understand" their significance ( Mark 8:21). As God had provided bread abundantly for the Israelites in the wilderness, Jesus had provided bread abundantly for them in another wilderness. The conclusion should have been obvious. Jesus was the prophet that Moses predicted would follow him and supersede him. He was the divine Messiah. [source][source][source]
"His rebuke was not because of their failure to grasp the meaning of His warning ( Mark 8:15), but at their failure to understand the meaning of His presence with them." [1][source]
It was extremely important that the disciples perceive who Jesus is. Without that perception they could not enter into relationship with Him that was realistic and fulfilling. Jesus" use of questions forced them to interact with the implications of what they had heard and seen. [source][source][source]
"In this way, Mark appears to say that being an "insider," even a "disciple," did not guarantee that one "understood" or perceived the significance of Jesus and his ministry." [2][source]
The incident ends with a question but no answer. Mark leaves the reader hanging. The answer is of utmost importance. Peter finally verbalized it in Mark 8:29. However the reader of this Gospel already knows the answer because of what Mark previously wrote. [source][source][source]