According to the author of the book of Nehemiah, this part tells us how Shallun, a leader, worked hard to fix and improve the fountain gate, making it strong and secure. It shows that many people helped to rebuild the city and make it better for everyone.
Nehemiah 3:15: But the gate of the fountain repaired Shallun the son of Colhozeh, the ruler of part of Mizpah; he built it, and covered it, and set up the doors thereof, the locks thereof, and the bars thereof, and the wall of the pool of Siloah by the king's garden, and unto the stairs that go down from the city of David.
Nehemiah 2:13 refers to Nehemiah inspecting the gates including the gate that was later repaired in 3:15, potentially contradicting the timeline or condition mentioned in 3:15.
Nehemiah 2:13: And I went out by night by the gate of the valley, even before the dragon well, and to the dung port, and viewed the walls of Jerusalem, which were broken down, and the gates thereof were consumed with fire.