I think that's a legitimate question, because really, a voluntary CPP is a big RRSP in which everybody participates. Why is that better than participating in an RRSP on your own? Well, there might be a couple of reasons.
First of all, if you're putting your money into a fund that is professionally managed, as the CPP Investment Board does now, maybe your fees will be a little bit lower and your investment results a little bit better. Because typically people don't choose their investments very well, and they don't do as well as large pension managers do with the money.
The other advantage is that you don't have to think about it.
But in essence, putting your money into a supplementary CPP is pretty much the same as putting it into an RRSP, conceptually.