Answer:
To save time paddling
Step-by-step explanation:
The excerpt I found says “See now that you pull the Sea twice a day and twice a night for ever, so that the Malazy fishermen may be saved paddling.”
From what I could find and read it seems because the man is too lazy to row home, the eldest magician asked the fisherman to create the tides to help the man and his children with paddling because they are lazy.