

They stop using GPS recievers, therefore nothing for GPS to connect to.
Its a slow transition, but it is one of magnitude. The US could shut down the satilites over Iran, like Starlink did it with Russia/Ukraine.
They are preparing for that, and mostly for the US to loose the power to potentially do it. It matters in case of all out war.
Yes, you should use it for the fastest lock, but if your interest is to keep the US out of influence, ofc they would do it.
Either that, or they do it to please China hoping for an ally. Second option might be their played hand, even if it doesnt work.