Afternoon Beech
1) A bloody big pot hole ( all too frequent) he needs to avoid
2) Cats, dogs, squirrels etc etc
3) Pedestrians
4) Cyclists
5) Puddle avoidance
6) Birds flying across the cars front
7) etc etc etc
All these hazards will be there whether you signal or not.
I was taught to regard 'unofficial' signals like flashing headlights to mean 'I am here'. You then interpret the other vehicle's behaviour to make your decision. If waiting to join the main road at a tee junction and any oncoming vehicle flashes his headlights, you wouldn't act on that alone would you? You would confirm that he was indeed slowing down and there wasn't something behind , just waiting to overtake him.
Similarly, if someone is indicating to turn (with an official signal) you'd wait till his speed and position confirmed it in case he's left the indicator flashing from his last turn.
I regard giving a following vehicle a single NS indicator flash as telling him that I know he is there and I am ready to be overtaken. Motorcyclists, in my experience, usually raise the fingers of the clutch hand as an acknowledgement after passing, so most of them where I drive seem to be aware of the practice. The decision to overtake rests with the vehicle behind, not an action of the one in front.
Cheers
Dave