I assume most startups want to grow their engineering headcount (there are exceptions, of course). The pickier/more-focused you are about what kinds of engineers you want to hire, the more you have to gain from casting a wider net geographically.
But if you find the perfect teammate and she can't come to your main country due to visa restrictions, or maybe even won't come because the immigration story isn't worth the trouble, then what are your options?
I'm not talking about money here, though of course that will be a factor for some people. It just strikes me that the "YC companies with offices in Canada" have a serious recruiting advantage over those without, once they need to seriously grow their teams.
At larger scale, it makes a LOT of sense and is done regularly.
Some times, it can be a hiring DISadvantage, as candidates run away from jobs where they need to wake up in the early hours for a conference call and also need to jump on conference calls at 11pm.
I do this currently, we have a dev team in the Ukraine. It is worth it price-wise, but the cost not accounted for is my personal time, i'm often on conference calls during kids' bedtime story telling.