It's not just a manga trick, but I suppose that is the most well known example of it.
There are a few reasons for it. First and foremost it's a stylistic choice. It's also production/time saver since I don't gain anything by drawing a full face on a character with a 10px by 10px head. Objects/people farther away in the distance don't need a lot of detail crammed into them because, at 72dpi on a monitor they would just look a mess. They style I use to draw the comic is very simplistic anyway.
I also use it sometimes when I want to show certain expressions with as little detail as possible. When I draw the face with just two circles for eyes then the expression is blank. Then the expression can be changed in a very obvious way with the mouth or eyebrows. Sometimes it can make a frown look comical, and others it can be used to show a non-specific expression which is up to the reader to interprete themselves.