I can understand why some people might find some redundancy, because the Criminal Code already criminalizes polygamy. If you are criminally convicted, you can be deported from Canada on that basis. But I think what people are missing is the fact that having an immigration inadmissibility sanction, you can put people through an administrative process on a much lower standard of proof and make it easier to expel people from the country than it would be if you put them through a criminal court.
Criminal prosecutions are time-consuming and expensive, and there are a lot of checks and balances. In many cases this is a positive thing, but putting people through an administrative process obviously facilitates people becoming inadmissible and removable from Canada. If that's the goal, then it certainly achieves it.