Commerce and drainage were the original goals of the canals.

Commerce was first. From the founding of the city shallow draft ocean going sailing vessels found access to the city through the lake was much easier than sailing 95 miles up river against the current. Bayou St. John provided access within two miles of the river. Canals and associated turning basins were added to provide additional capacity. A system of railroad lines provided for the overland portage.  Providing shipping from the founding of the city in 1718 until the 1940's the canals have had a profound impact on the city.

Commercial use has become restricted to a few of the larger waterways (Industrial, Intercoastal, Harvey).

Drainage remains a critical function of all the canals.

Wikipedia for more info