`=canal` is used on `bridge=aqueduct` , which would be used for  water supply as well. A catchwater doesn't carry excessive useless water away, unlike drainage tunnels. It intercepts the streams to redirect them to the reservoir, or even the Settlement Basin here directly. The water hasn't become useless, as it will never reach built-up areas. 
Also an unlined "ditch" can be used for irrigating rice paddies and farm fields, especially in Asia. But `=ditch` is for drainage only.  So the words shouldn't be taken literally. At least in HK common language, "canal" usually refers to water channels for ships, not irrigation channels common elsewhere and used for `=canal` in OSM. 