To signify it was made up of persons from all parts of the world. This agreeth with Ezekiel's vision, Ezekiel 48:31-34. The triumphant church will be a collection of believers, who from all nations have come into it.
Related Commentaries of Revelation 21:13
Revelation 21:13
13 On the east three gates; on the north three gates; on the south three gates; and on the west three gates.