IRCC can send you a letter asking you to show up at a particular office three or four weeks later (interview, test, oath). If they know you aren't in the country, they will wait with the invite accordingly. If you don't tell them, that authorized person might receive the letter saying you have to be somewhere in three weeks. That is of zero help if you happen to be at the other end of the world at that point. That's why they want to know when they can't invite you to things.What about someone who is studying outside Canada,has required days for citizenship. There is no change in address, the person is still living in Canada and PR for all practical reasons including filing of taxes in Canada. Staying in a dorm cant be called residence. Such people will apply for citizenship and leave after the break is over and come whenever the test and oath is due.
Is the objective of informing IRCC just to help them to reach the applicant? If so, the person can authorize someone to receive mail. Isn't that possible?
Also, let's not get into the topic of residence for tax purposes. That's a whole different area and independent of all the other definitions of residency.