Because (at least on my Moto X 2012, Android v5.1 Lollipop) rebooting can take upwards of ten minutes. Some OTA updates apparently trigger an 80-some-odd module "optimizing" run, which I presume is ART doing its AOT thing. If you're in a hurry to use your phone this can be maddening.
Never had that happen on any of my Nexuses (S, 4, 5X), outside of OS updates (which don't happen without you approving them).
One of the reviews I saw said the rebooting time was pretty fast
Yep, ART really trashed boot time.
My LG G2 reboots in one minute. I would hope the G5 would be about the same.
My Note 3 takes about 25 seconds to boot up completely. Newer devices tend to be faster booting up, so I don't think it will be a problem for the G5.