I understand the need for guide dogs to assist the blind, but why do some people without canes have them?

Either-Or, not both: Service dogs require the use of one hand, and the other hand is kept free to interact with environmental obstructions such as doors. Leader dogs are way more effective than canes in most cases, because they think! the dog will keep his or her companion out of harm's way. A cane can notify the user of obstructions and help in ambulation but can't stop someone from walking into traffic!