The idea is to establish the habit, so that you do it automatically every time, including the relatively few times where it really matters.

It's easy, it's never harmful, frequently helpful, and occasionally saves lives.

I ride motorcycles, so I have had more opportunity to think about the importance of turn signals than others might. But I also drive cars, and I believe the benefits are universal and have zero negative consequences, so I include this as a minimal-effort suggestion.

I actually sort of agree with always using turn signals, if not really the other point. There are a lot of behaviors that make sense to do reflexively because, if you have to think about whether to do them this time you may forget or incorrectly decide not to another time. Presumably you wouldn't think it was OK for them just to go through a red light because they don't see any other cars around.