Because genius what else would you recommend selecting comm 2 to? A random CTAF frequency? Ohhh perhaps numbers?
We use comm 2 for ground frequencies, CTAFs, and to monitor 121.5 in flight - once on tower on comm 1 I have comm 2 queued up with the ground frequency, if there is one, but muted, so it is just as easy to switch to that as it is to switch the flip-flop on comm 1.
The comm 2 antenna is on top of the aircraft rather than underneath it and is optimised for communications when the aircraft is on the ground. It makes a huge difference at times.