“Immediately Jesus made the disciples get into the boat and go on ahead of him to the other side, while he dismissed the crowd.”

The word “immediately” means that without any time to wind down after this amazing miracle of the feeding of the 5,000, the large crowd on whom Jesus had compassion (verse 14), were dismissed, and the disciples whom he had been instructing were sent off, away from the crowd, by themselves in a boat.

The reason for this is given in two references:

Matthew 12:23 – in response to the healing by Jesus, “All the people were astonished and said, ‘Could this be the Son of David?’” (the military conqueror they sought to free them from Roman rule).
John 6:14-15, (after the feeding of the 5,000), “After the people saw the sign Jesus performed, they began to say, ‘Surely this is the Prophet who is to come into the world.’ Jesus, knowing that they intended to come and make him king by force, withdrew again to a mountain by himself.”

This Jewish crowd knew their scriptures enough to know what they wanted – a political warrior – but were blind to what Jesus was showing them, and they were sent off to stop their intentions proceeding any further. Perhaps the disciples were sent separately so they would not be affected by the views of the crowd.

In contrast, the gentile crowd in chapter 15, which records the feeding of the 4,000, responded to the healing miracles of Jesus with (15:31), “And they praised the God of Israel.” We need enough bible knowledge to respond with praise for the faith and salvation we have been given, but not so much as to create our own labyrinth of regulations and doctrines that too easily become self-serving.

And looking forward, we see in verse 23, “After he had dismissed them, he went up on a mountainside by himself to pray”.
The other reason for this dismissal was for Jesus to have time alone in prayer with his Heavenly Father – perhaps to regain focus on direction after he had rejected the temptation to become an earthly king.
It is so important for us to have time in prayer with God, to be freed from earthly distractions and to know His path for us.

The main blessing of prayer is not for us to ask God what we want or to change what God may do, but that by taking all our concerns to God He will more closely align our thoughts and concerns with his purpose for us, to change us and what we may do.