why was it important for the swallow to migrate?

The Swallow has a strong desire to migrate to Egypt because that is where all his friends, the other swallows, have gone to stay. The Swallow has only stayed behind because of being in love with a reed. But she rejects him, so he longs to join his friends. He dreams of being in Egypt with his group, as he tells the Happy Prince statue:

“I am waited for in Egypt,” said the Swallow. “My friends are flying up and down the Nile, and talking to the large lotus-flowers.”

Despite the allure of the many beauties of Egypt, the Swallow pities the Happy Prince, who is so sad because of all the suffering he sees in his city. The Swallow stays on and on, helping the prince to alleviate the miseries of the poor.
Because he gives up the pleasures of being with his friends in Egypt, we are able to see more clearly the sacrifices the goodhearted Swallow makes to help those who are in need.