For me, this feature was a godsend. When I’m on call I couldn’t use iOS’ traditional Sleep and Do Not Disturb mode because they would block all SMS and there was no way to override that. Moreover, with Focus settings, I can allow through the notifications for HomeKit and my security cameras while blocking other notifications.
Being able to set up separate Sleep and On-Call Focus modes that allow me to selectively allow calls and SMS through from random numbers (our on-call calls and messages can come from any number) only when I’m on call has completely changed and greatly improved my experience with iPhone at night.