The point of eu is less about geographical location, and more about increasing cooperation and trade. It’s evolved to more than that, but Canada joining is no different than any island nation already in the eu. Ireland, Cyprus, malta are all seperate from continental Europe. Canada trades heavily with the USA, but that relationship is not currently positive. The USA would have less ability to force conditions on Canada if they were part of the eu. Instead of being the smaller trading partner beholden to America, the become part of an equally larger bloc. No tariffs would be possible without tariffs on all. And likewise, reciprocal tariffs would be imposed by all states. Trump in his first term tried to negotiate terms with Germany. He couldn’t grasp the collective nature of the eu when Merkel deflected him on multiple times.

I think there is likely less upside on trading for Canada, than, say, Iceland or turkey, but there is still benefit, as a smaller country beside a larger one, that’s their biggest trading partner. Those benefits are minimal for Iceland, but moderate for turkey. For each country it’s a different list of pros and cons.