The law requires you to signal even when you don't see any cars around is true.
Even if you don't see any cars around there could be cars or a car that suddenly comes up behind you or in a lane ahead of you.
So the law states that you must always use your turn signals when making a turn even if you don't see any cars around.
You must always use your turn signals when changing lanes as well even if you don't see any cars around as cars can suddenly come up behind you.
Always use your turn signals when making a turn or changing lanes.
I've always got into the habit of turning my turn signals on and it's a good habit that I can't break which is good.
Even just driving in my small town where there really are no cars usually around I'm always using my turn signals.
If you fail to use your turn signals even when no other cars are around and a cop happens to show up or be watching and notices you could get ticketed.
The law does require you to use your turn signals when turning or changing lanes even when you don't see other cars around.
This is also because of blind spots.