Well, 代理人 is just a representative, and it could be a family member, a friend or a lawyer, an agent if you hire one.
the result of leaving it blank is that you will be refused to get involved in case processing when you need to. for instance, make a call to the call center in purposes of asking questions or verifying something. however, it would never be too late to submit you representative form at any time during case processing.
it is espacially important and useful when parents apply for extention in Canada since most of our parents don't speak English.
In the case of applying visit in China, there are chinese employees in Canadian embassies who usually help a lot in communication.
by the way, it is worth to be prepared just for in case.