Take someone whose refugee protection claim was denied. CBSA has an obligation to remove them from Canada as soon as possible. Submitting a complaint could signal that the file needs to be updated, prompting the officer to conclude that the file is ready and the person needs to be removed from the country. That alone puts pressure on the person not to complain.
If a problem is reported and nothing is done, the fact that a third party can bring the complaint to the administration and a commission provides another complaint access point. That's a straightforward, but common example.