Crown corporations exist because it is felt their work should be kept at arms length from the government. A politician is forbidden from interfering in the daily operations of a crown corporation. Thus a politician cannot tell the CBC to report a certain way and cannot tell VIA Rail to start servicing their community.
The legislature only has the power to set the budget of the corporation and to appoint its chairperson. This gives a fair amount of control, but corporations are free to speak out against the government, as the CBC frequently does.
There are two types of crown corporations. The first, like the CBC and the Canada Council, exist to advance cultural goals that the private sector might not do adequately on its own. The other type, such as VIA Rail or the Saskatchewan Wheat Pool, are natural monopolies that will not operate to the benefit of consumers in a free market. Many of these types of crown corporations were sold off by Brian Mulroney's government in the 1980s.
Canada has a number of crown corporations. These include: