Truck Scales on Canadian Highways

I have always wondered about the truck scales on the sides of major
highways. I would like to understand how they work.

Does a truck have to report to the scales once a trip or every time it
passes the scales? Once they find out how heavy the truck is, do they have
to pay fees based on the load? What is the reasoning?? Is it some kind of
user pay or is it a way to help moving vans measure the weight of their
load? Any explanations would be appreciated!

Thanks!

Joy