At the time it was proposed, shortly after the War of 1812, there was a real threat of attack on Canada from the United States. The possibility of future American attacks had to be provided for and if possible, deterred.
Its initial purpose was military - to provide a secure supply route between Montreal and Kingston, Ontario. So westward from Montreal, travel would proceed along the Ottawa River to Ottawa, then southwest via the canal to Kingston (and vice versa for eastward travel from Kingston to Montreal). The key being to bypass the stretch of the St. Lawrence River bordering then Upper Canada (now the Province of Ontario) and New York State which would have left British supply ships vulnerable.
Locks entering the Ottawa River
In winter, the section of the Rideau Canal which passes through the city of Ottawa becomes the world's longest skating rink. It serves as a popular tourist attraction and recreational area and is also the focus of the Winterlude festival in the City of Ottawa. Beaver Tails, pastries topped with ingredients such as cinnamon, lemon, and maple syrup, are a popular treat on the Canal and are sold, along with other foods, in kiosks scattered along the length of the skateway.