As a nightstand spare, carry spare, keep in the car spare, travel spare... having higher capacity available is ALWAYS a good thing. Not something I would carry in the gun with.
This article lists lengths with the different mags.
Glock 17 height is 5.47" for 17rds. Relevant? The difference between the 10rd flush and the 12rd is occasionally enough to carry with the 10rd. The 15rd is just over .75" longer than the 12rd. I'll end up with a couple of them eventually, as they have lots of uses. But concealed carry would be awkward with the short barrel and long grip.
The 15rd would make more sense to carry with if one was using the P365xl. The side profile would be very similar to the G45 with the longer barrel/slide and the 15rd mag. Just significantly thinner. Hmm...