I also don't understand and I don't think it was a good idea. In my opinion it totally removes the glamour and protagonism of a country as the registered brand of the World Cup's edition. When we talk about previous World Cups it's nice to attach each of them to a different country, as the culture of the countries which host the event become a symbol of their respective editions. Now with Canada, USA and Mexico hosting a World Cup, all of them at once, it becomes a confusing event with no original and memorable traits at all. Maybe they did this to cut expenses for the hosts, so the costs are splitted between the three parts, but strange anyway.
It's not the first time that this was done though, Korea-Japan 2002 comes to mind, and it was quite successful without any major issues.
Now, this one will definitely be a step forward and will cover a vast amount of distance, plus a third country, so we'll see how it goes.
But in any case, almost all the games are going to be played in the USA, so only a minority of matches will be hosted in Canada and Mexico. This means that not everyone will have to travel to three countries.