[答疑解惑] It’s a good job I came when I did什么意思呢

Biff and Chip hung on to the rock. The water roared past them. Biff and Chip were frightened. “What are we going to do?” shouted Biff. “I don’t know,” called Chip. “It’s too far to jump to safety.”
Then they saw Kamar. He had followed them to the mountain. “Hang on!” he shouted. “I’ll soon have you safe.” He threw a rope across the gully. It caught on a branch on the other side.
Biff and Chip climbed to safety on the rope. “We are so pleased to see you!” said Biff. “I thought we’d never get off that rock.”
“The evil genie did this,” said Chip. “It’s a good job I came when I did,” said Kamar. “I came to help you rescue Floppy.”
It’s a good job I came when I did什么意思呢