Why do people hang up papers on their doors? This has been puzzling me for a long time, I have seen professors and other academic people hang up their research papers on their doors. What is the purpose of this?

My first guess was they were announcing some important papers related to a class they are lecturing, so students can look at them without disturbing professor. This doesn't feel right because there are not many papers studied in a typical class and there is internet to announce these things.

As the time passed I started thinking it is a way of advertising. But, this also doesn't make sense because web pages exists to serve this purpose and few people come to visit the office physically.

Maybe this practice is limited to my country, but any answer is appreciated.
In addition to the reasons mentioned by others, I think there may be something of the big-game trophy hunter about the practice:  "Behold, visitor, I am a mighty scientist, capable of hunting the fearsome [prestigious journal]!"