The program is not limited to YYZ. So perhaps you should be asking not what the point of the program is, but what is the point of YYZ participating in the program?

Ie, not all locations are participating in Select & Go. Most of those that can't do it (for example, because they're too small) don't try.

However, just because it fails at one time of the week does't mean it fails all the time, even at that one location. So you have to expect to be "the blind men and the elephant" (you can't tell if what you're seeing is at all representative of the whole program) if you're always renting from the same location at the same time of day on the same day of the week.

I can imagine, for example, that they might have a staffing issue in the hours leading up to you arrival time, and there's just no one available to stock the Select & Go aisle at that hour (unless they resolve the staffing issue).

Going to the Preferred desk and asking if you can get another car can't hurt anything other than possibly waste your time (if it also gets no results), so you might as well try. Keep in mind this was the only way of getting a car other than what you were assigned before Select & Go originated a handful of years ago.