I think it's about politics primarily.
The four current member states may find some economic advantages among themselves if they can harmonize more, but I see the Pacific Alliance as primarily a political move to band together as a group of like-minded states to find reassurance in each other's company, and as I said, to balance against the more left-leaning regional trade groupings.
Goal number one is to group together against Brazil and Mercosur and ALBA. On the other hand they are interested in opening up to Asia Pacific as well.
On the Canadian side I find the idea puzzling, but I would venture a guess that the government wants to find ways of signalling, in Pablo's terms, that we have friends in the Americas, which is good. But as I said, I worry that politically that might isolate us vis-à-vis other countries in the region we might also want to be friends with.