If someone doesnt stand in my healing-bubble.
If I know that someone wont make it out of an AoE (or stands in a generally stupid place, like dps in front of the boss - in which cases it gets paired with an explantion afterwards like "Dont stand there, its dangerous", sometimes you just dont have the time to type the explantion and wait for the person to move accordingly, Rescue will do the trick much faster).
If I know that someone is animation locked.
If someone runs away with a stack-marker - again followed by an explantion, but that might be to late, Rescue will save them.

During raids my static-healers also used Rescue to drag tanks and melees out of huge AoEs to allow for more uptime - as someone who tanks those fights I always appreciated that.