The restify API Guide has this to say:
You are responsible for calling next() in order to run the next handler in the chain.
The 'chain' they are referring to is the chain of handlers for each route. In general, each route will be processed by multiple handlers, in a specific order. The last handler in a chain does not really need to call next() -- but it is not safe to assume that a handler will always be the last one. Failure to call all the handlers in a chain can result in either serious or subtle errors when processing requests.
Therefore, as good programming practice, your handlers should always call next() [with the appropriate arguments].