It's to ensure that everyone gets the same-size lot. The closer you get to either pole, the more lines of latitude converge; a vice-versa as you approach the equator. In Canada what that means is that if you start at, say, Eglinton and draw two lines due north, by the time you reach Lawrence, they're slightly closer to one another than they were a at Eglinton. It may not seem like much but it does add up and it's not long before you're short-changing people a quarter of an acre. So, periodically, little adjustments are made in the concession roads so that the distances, and thus the lots that they define, remain constant. In the days of horse-drawn carts or a handful of cars an hour, doglegs like this aren't all that bothersome. You'll still find them if you drive far enough up in York Region. But as you get to the point that several cars a minute have to negotiate these turns, that's when municipalities tend to smooth them out. Drive north in Scarborough or North York and you'll notice broad sweeping curves as you approach major east-west streets like Sheppard and Finch (for example). When you do, you'll know you've reached one of those former doglegs.
They do happen on east-west streets, but less often. In those cases they usually represent different surveys, or a kludge around some other anomalously in trying to create fair and equal lot sizes.
