One point no-one's mentioned: a fresh signal may convey more meaning than an old one.
I'll explain.
Imagine I'm waiting (on foot, on a bike, in a lorry, doesn't really matter) to cross a road or a junction. A car approaches round a bend to my right. If the car is going to turn left before it reaches me, it'll be safe for me to cross. If not, it won't be.
If that car has its left-hand indicator flashing merrily away when it first comes into sight around the bend (because the driver, despite not being able to see whether there's anyone who might benefit from the signal, has helpfully switched it on anyway before he could even see the turn he was going to take), then I don't know whether he's been driving around with it flashing for the past half-hour, or if he really is going to make that turn.
If, as the car approaches, the driver sees me (possibly making eye contact in the process), and then puts on the indicator (a 'fresh' signal), I'm much more likely to believe him. If I'm a prudent person, I'll probably wait to see if he slows or changes direction as well, but the fresh signal has given me more information than the old one would have.