I use it for all of the above, but the most use of all is to get people out of the left lane if they are going to slow, below the speed limit...
Driving in Europe is a great pleasure, drove for 4 years over in Germany, Its funny We (the U.S.) have the same laws for driving (or they are very similar) here in the states about passing in the left lane and staying out of the left lane, they are just not enforced... go figure... more laws we have in place that are not enforced...
If you were in the left lane driving 80 MPH, and a faster car came up behind you and flashed his lights at you, Which they did from a great distance away, you would notice his lights and you got out of the way quickly, you would not try and pass another car and then get over,,, you got out of the way, because chances are he was running almost twice as fast or faster than you. And they will hit your sorry butt if you do not get out of the way... seriously... saw some wicked wrecks over there because someone would not get out of the way!!! That somone was usually an American driver that was the cause the wrecks, (easy to identfy due to the different vehicle tags they had)
Also Hand gestures of any type are illegal, you gave someone the finger over there or made any type of hand gesture to another driver, you can bet that the Polezi would find you sooner or later and give you a ticket for it...
I loved it over there, wish I could go back...