Its about stacking the deck. If there's a mandatory industry, then we'll be guaranteed to be able to compete with hollywood for the Canadian audience. With enough content, the trial and error to finding success is more likely. Once a production company gets a little bit of success, they can grow, thereby encouraging Canadian arts. Writers, actors, crew members, designers, etc. would have to move to Cali or India to find work otherwise if Canada had to compete with these markets without assistance.
Likewise with the music industry. If companies have to find Canadian artists to put on the air, it creates a market. Companies don't want to belly-up so they have to find enjoyable arts. Some even goes as fsr to create them.
This allows us to be exposed to less "constitutional" ideas that's not Canadian, historical events and whatever. Plus there's the economics of exporting content rather than importing it. So a few baseball and NFL fans have to suffer for the good of the CFL and Rugby? Good thing policy takes into account the needs of the majority.