You're leaving for work. It's dark. It's cold. It snowed last night and your car is blanketed with several inches of powder. Do you:
a) Just give the windshield a swipe and hope the rest blows off before you hit the highway?
b) Get your snow brush out and thoroughly clean off your entire vehicle, cursing our Canadian weather and dreaming about the giant cup of coffee you're going to buy on your way to work?
c) Go back to bed?
If the answer is b) or c), you're good to go. But not thoroughly de-snowing your car is not only dangerous for you and other drivers, it's actually illegal. Fines for not completely cleaning off your car vary by province, but you can be pulled over and ticketed by police if they deem your vehicle insufficiently clean.
Recently, in Ottawa, one police officer stopped 63 vehicles in the course of 90 minutes because they hadn't been cleaned properly. Lucky for the drivers, he let them off with a warning - but not before he personally finished cleaning off their vehicle.
Leaving snow on your car creates a danger for other drivers - if snow or ice blows off your vehicle into their car, they could end up momentarily blind or with windshield or wiper damage. Winter driving is hazardous enough as it is - adding snow and ice flying off your vehicle as well increases the risk of accidents for everyone on the road.