Perhaps it was maintained in them more as a legacy function, like AI-Focus which has long been considered borderline obsolete.
The apparent purpose seemed to be, if you were shooting a fast moving, once in lifetime subject like a rarely seen bird, or a space shuttle take-off, would you rather have the camera prevent shutter operation until focus was confirmed, possibly missing the shot altogether, or choose Release priority where a burst would hopefully result in a capture even if not in perfect focus.
Apparently, all of the continuing Servo AF, & Tracking advancements in the "R" MILCH era have rendered that function obsolete