Rural Ontario requires its postal service. Many of the places in rural Ontario do not have reliable Internet. The post office becomes an economic development tool for a downtown. If people have to come downtown to the post office, they might be more likely to stay in the downtown, and that's a downtown revitalization tool.
In the area where I live, I'm in a fairly large municipality of 35,000 people, and yet in the part I live in, it's rural. There are a lot of farms and a lot of seasonal workers. They ride their bikes to the post office to wire money home to their families.