>If "just go to the police" were an option for all domestic abuse victims, I'd expect that there'd already be plenty of other blinking neon signs they could point to.
Not really. Proving domestic abuse is not easy. It's mostly "he said, she said" until somebody ends up in the ER. That's why restraining orders are so common, because the evidence available won't lead to a conviction, but the legal system still wants to provide some kind of protection to victims, even though the protection afforded by a restraining order isn't that reliable. A big limitation for victims in going to the police is that if there isn't evidence to convict, the system does not effectively protect them from retaliation.
Now, on the other hand, if you can produce a device with some software illegally installed on it, you've got physical evidence that can be presented in front of a jury leading to a felony conviction. It's easily more reliable than most evidence used to prove abuse, which tends to be testimonial.
A remarkably high proportion of abusers walk away even when the police have been involved, cf Brian Laundrie and Gabby Petito. So no, there are not plenty of blinking neon signs. There are almost never blinking neon signs.
>The difference is that the phone doesn't sprout legs and run away from the victim, or otherwise assist the abuser in taking it away.
This is trivial and irrelevant.