The reason would be to have the same power indoors and out. How do you know your PM and trainer are the same all the time, have you graphed both measuring them separately?
Typically the slopes of each device will be slightly different, meaning that the could be a match at 200 watts, but low at 150 watts and high at 250 watts.
All that being said, my Kickr Snap is surprisingly spot on with my Quarq PM, as I’ve done extensive comparisons, see below link;
I used to never use powermatch because of that. However, I’ve come around and now always use powermatch except for the ramp test. Yes, powermatch feels different at times during workouts compared to the trainer’s Erg, but I feel it closer represents the fluctuations of outdoor riding and I know I have consistent power numbers inside and outdoors.